Well the miner is run on their (currently) closed source firmware.  I don't think Bitmain would ever purposely brick a unit, but who knows, there could be a disgruntled employee, or a coder who knows an exploit.  There could also be an exploit someone has found in the code that affects all Bitmain models and they decided to test it out on a new released miner.  In the latter case all that would probably need to happen is for the hacker to gain access to the pool software.  There are a bunch of different scenarios, none very likely.  I'm just saying I don't think there is anything a pool could do to brick a miner without knowing some zero day exploit.

But lets just put it this way.  If a company like Intel can manufacture processors for a decade containing major exploits that go unnoticed, its not impossible for much smaller companies to repeat the same mistake.