Heavy bullet with not enough twist.
I used to have a Rem model 7 in 223. It had a one in twelve twist rate. Shot nice groups with 40-55 grain bullets. I loaded up some 68 grain bullets and they all tumbled. Now I shot a savage in 223 with a one in nine twist rate. Haven't tried any light bullets, but with 55 grains and up . . .it's tack driver. I mean really.
Live well, be safe.