If someone breaks into your computer and your Coinbase Pro, the closest thing in my mind would be to withdraw the coins to another wallet and disappear . But in the OP's case, someone broke into only to place orders, this is more than weird and nobody will use so much resources to just fool around with your orders.
Looks like not everyone was reading the OP deeply enough.
I wish there's more investigation on that case.
it would help to know which orders were filled/which coin was bought. that would probably explain things.
the nature of this case reminds me of the high profile binance "hack" where phished logins/API keys were used to pump viacoin. the hackers used compromised accounts to buy up VIA, while they used dozens of other accounts to sell VIA at the top. then they tried to immediately withdraw the BTC. https://hackernoon.com/alleged-hack-of-binance-linked-to-viacoin-pump-bb9066bf96bf
exactly, that's why i'm curious which coin we're talking about. this could have been part of a larger operation. in the VIA case, it was a very low volume market.