Credit goes to Spartan at the NerfSG forum for detecting and solving this issue.
Some users who have modded their Nerf Stampedes may have encountered an odd issue whereby their blasters continue firing even after they release the trigger, resulting in erratic firing operation... or the bolt moving to and fro but the catch not releasing at all.
Here is a possible cause of this phenomenon:
The Stampede's Plunger Catch Control Spring has somehow popped out of position and no longer functions properly.
The solution is to remove the catch control component and reposition the spring back to its original position, then install it back into the Stampede.
Applying some tape to cover the gaps can also help prevent the spring from popping out again too.
This should help solve the Stampede's "auto-running" or erratic firing issues... or at least narrow down the causes. :)