Because in order for the proof to succeed, Lean needs "strange" assumptions about the code. This rare assumptions are the conditions to trigger the bug.

Imagine a proof that needs the axiom that all numbers are below 2^32. What happens if my system is expected to handle numbers greater than 2^32?? There you have the bug... Also it could mean that the system is not validating the boundness of the input, and 2^32 is the maximum that allows for coherent execution downstream of the execution. In both cases, it needs to be checked :)