One of the reason some well off traders have multiple accounts is because they will use one or two to concentrate their most speculative positions (overnight holds) and another one or two for their more conservative positions (daytrades, spreads etc.).
They will never keep significant sums over their margin requirement in the speculative accounts so that if they blow up the broker does not have funds to offset against. If there is a blowup and they owe the broker $100,000 they offer to give them $60,000 for a general release of liability. Since the broker will probably have to part with at least $20,000 to a lawyer, take a hit to their capital and wait many months to be made whole he'll come back with a counter offer of $80,000 and probably take $70,000. The scum bag customer saves 30 large.
It sucks that men of substance will calculate leaving someone else with the bag that their decisions created but it is a technique that has been used forever.
And, to answer the OP's question: You are responsible, as the British say, right down to your last cuff link.