I purchased a used but very clean P229 in 40ACP recently and could use some advice on identifying and correcting a small problem.

What would cause the gun to decock after firing?

I fired maybe 50 rounds rounds through the gun when I got it and it worked fine. I then installed the Sig SRT kit, and since it was a used gun decided to give it a new spring kit while I had it apart. Reassembly went well and when I went to shoot it the hammer decocked 4-5 times during a 100 round test. Everything about the gun feels right and the SRT works great.

When it decocked after the shot it fired the next round in DA as expected.

Since it's a trade in, and I only had maybe 50 rounds through it before disassembly, I can't say for sure if the problem existed before I replaced the springs and installed the kit. The gun is otherwise flawless. Not a single FTE or FTF.

Any help would be appreciated.

Damaged hammer or sear, broken sear spring, or sear spring disengaged from retaining pin.