Serious Organised Crime Agency

it could be because some of the people that you had dealings with where under investigation themselves or with the case of the 100k cheque it could have been forged / stolen funds. usually the banks would report dodgy goings on, sometimes they even know whats going to happen before it does - banks have seriously good intel regarding frauds and scams going on, usually for the local area, but if somebody had tried to pay in a stolen cheque then it would be a massive red flag and all accounts related would be frozen.

if your account shows unsual transactions they will report it, they might suspect you being a money mule i dont know, but it does sound like something bad's happened, if you've not done anything wrong then you've nothing to worry about although it will be more of a major inconvienece than anything, and police might want a word with you. i'm not too sure what happens after the account closure, my experience of dodgy goings on at a bank are we got told from head office that things had been reported to soca and after that you'd stay away from said account because it would be monitored, but they'd then be closed and it would be outside the banks control - at least at branch level, short of the risk assesment stuff we always had to do i didnt have a great deal to do with it so i cant say what might be going on in the background.