does anybody understand why my ex spouse has transferred a significant sum from his private US account to our still existing joint account (including the fee for transfer) and from the joint to an overseas account? Will that trigger tax implications which may effect me too?
Difficult to answer. He could be thinking that this will accomplish something that it doesn't.
Two possibilities come to my mind: 1. That there was some reason why he couldn't do the transfer from his own account, or why it was inconvenient. Like the bank where he has the new account doesn't provide a way to send money overseas from their web site but the bank where you have the joint account does let him do this. Or his new bank has higher fees for such a transfer than the bank with the joint account. Etc. If that's the case, and he's not trying to take your money, then it's a big "so what".
- He's hoping to make it look like you made the transfer, because he's hoping to stick you with the taxes on something or otherwise do something shady.
If you're separated or divorced, why do you still have a joint account? Why not close the account, split the money, and go your separate ways? I wouldn't want to have a joint account with my ex-wife. It would leave me wide open to her taking any money of mine that I put in there, her overdrawing the account and leaving me on the hook for it or at least damaging my credit rating, or all sorts of potential scams.