Before my retirement I had been intimately involved in the Iowa legislative process off and on for 36 years. Among other tactics, opponents of legislation may offer a myriad of amendments designed to kill a bill. A 60/40 majority is extremely difficult for either party to overcome even with a poison pill amendment. I don't know if the D's walked out because they couldn't get any amendments, but they certainly knew they could not succeed in their efforts to weaken the bill. Walking out may just have been their (perhaps risky) political statement.
NRA Life Member - RMEF Life Member - Vietnam Vet