zincwarrior: Has this happened to other people? Is this common? I'm not saying I am going back but its a definite surprise.
Yes, it is common...and aggravated by the fact that many modern 1911s come from the factory with extended combat safeties. Models equipped with ambidextrous safeties make the outcome even more likely.
Old school Standard (original Colt) safety levers are much less susceptible to the issue, although it can happen with them as well.
I've carried 1911s of various configurations for CCW. LEO, and Military use since 1975. Extensively. I'm pretty much a life-long 1911 user. In recent memory, I carried one on average for about 16-17 hours a day during three tours to Iraq (2007-2011). I still carry one today for civilian CCW. Years ago, in tune with then current gun fashion, I would have my 1911s configured with after-market ambidextrous paddles, during a time when factory guns didn't come with that option. I gradually grew to dislike the feature.
I've found that 1911 ambidextrous safeties (especially those with extended lever paddles) habitually disengage due to external pressures against 1) the safety itself or 2) the holster body (which then transmits that pressure, flipping the safety off).
This has tended to be a reality in my world whether I was carrying IWB, OWB, Belt Slide, Flap, Shoulder, Belt Clip, Mexican Carry, Vest Mounted, Belt Duty Holster, or Drop Tactical Hard-shell. It has happened whether I was using holsters made of Kydex, cowhide, horsehide, ballistic nylon, or hard polymer.
One of my worst offender holsters is a Safariland 6004 hard shell tactical holster worn at various times either on the thigh or low-ride off of a duty belt. That superb holster does not flex, but it still allows "things" to engage against the safety from the open top of the holster. And any pressure against the hammer area or butt of the holstered gun can move the gun slightly, causing an extended safety to rub against the inside of the hard shell...and disengage.
I had an old DeSantis OWB Speed Scabbard back in the 80's (when it was a cutting edge design), that I used for CCW...the safety would come off. Likewise with a favorite Galco OWB that I wore overseas. Likewise with a GI M7 leather shoulder holster...and both GI 1916 and Bianchi M12 flap holsters...although safety disengagement is a fairly rare occurrence in a flap design.
The only guaranteed solution I've seen is a Kydex holster with a molded channel that fits over (and immobilizes) the safety lever (in the Safe position). It ain't moving until you clear the holster. The version I saw was a DIY example by a holster builder on another forum. Brilliant work, but not commercially available.
The 1911 just provides more opportunity for the safety to move because of it's design. When carried, any downward angled pressure against a large safety paddle only has to overcome the safety detent friction that keeps it from lowering. It has a very small arc of travel between "Safe" and "Fire". By way of comparison, I've never seen the much smaller and stiffer safety on a BHP accidentally disengage. Nor the lower profile Safety/Decocker of an M9 Beretta, which also requires a larger arc of movement (against a stronger spring) between functional positions.
On the other hand, that very 1911 safety is so ideally set up for in-hand manipulation and weapon firing, that I can certainly live with the occasional holster hiccup.
My 1911's safety (on various guns) has disengaged while rappelling down buildings or off of helicopters; while mounting and dismounting armored vehicles; while driving up-armored executive protection vehicles; while patrolling across forests, deserts, jungles, and mountains; while conducting live assaults and raids; while conducting foot chases; while climbing over fences, windowsills, and rooftops; while wrestling with perps, fighting people in bars, and chasing folks through alleys; and while simply passing through doorways, tight passages, or bumping a hip into furniture. It has happened frequently during the simple act of fastening and wearing seat belts in all manner of vehicles and aircraft. I've had 1911 ambi-safeties flipped off just in the normal course of sitting in various chairs in offices. I've had that safety become disengaged while hitting prone under fire or low crawling. It has happened to me in training, both on flat ranges and in shoot houses. And I've had that safety get disengaged by some lovely's hip on a crowded elevator or a too-close bar stool.
No big deal, and I just flip it back on. I understand how a 1911's redundant safety features work...I simply prefer that my manual 1911 safety stay put. Standard single side safeties mostly do.
A lot of this has to do with actual holster profile. A lot has to do with physical activity during wear. And a lot has to do with body type. Love handles tend to press on 1911 holster sweat shields (or safety paddles) from the inside. Mine do (and I'm not anywhere close to being a trophy Dunlop Disease sufferer).
As you noted, you still have a well protected trigger and a functioning grip safety. The first time that event occurred in my holster, my hair stood on end when I discovered it. Nowadays, I just periodically check it throughout the course of the day and drive on. No big deal.