The fluid that is used to lockup the converter originates at the 1-2 shift valve...so there can be no lockup until after the 1-2 upshift occurs.

In normal operation, as the vehicle slows to a stop, the trans downshifts directly from 3rd to 1st bypassing 2nd. If there is a governor problem and the trans doesn't downshift to 1st just before the stop, the converter can remain locked up. Shifting to Neutral restarts the sequence.

Other problems could be the sealing rings on the input shaft allowing line pressure to lockup the converter un-commanded.