That's certainly possible, namely that the TA did something bad with some other account, and hence why I advocate that the OP continues to pursue the matter since BA will know that those seriously guilty of malfeasance would want to vanish. This is close to $5,000 worth of Avios so it's a non trivial matter. But notwithstanding the attorney's perspective, using a TA is unusual but is like using a personal assistant (PA). And just as the BAEC member is ultimately responsible for their PA's actions, so here too for the TA. It would not surprise me if the only thing the OP is guilty of is naïvety but that tends not to land well in this area, unfortunately.

What I will point out is that usually BA and AGL are fairly merciful when it comes to scams, hacks, keylogging and the like, even if the BAEC member can be said to have contributed to the problem (e.g. sharing passwords between different applications). So for BA to delete Avios and TPs suggests that there really is something really bad happening somewhere in all of this - they have been fired as a customer, which is their prerogative but doesn't happen often. The trouble is that BA never discloses what they know and here I suspect the OP will also be completely in the dark too. I hope they are able to recover something from this. But erasing TPs is fairly drastic and a clue to the problem. The OP is still able to travel on BA, but I don't think as things stand that they will be allowed into BAEC.