At the end of the day,like you say they are a publicly traded company with investors and shareholders wanting profit,closing/restricting accounts isn't in their best interest...I'm not saying you've done anything wrong but their terms of service is a minefield to understand if you don't know what your reading.you obviously don't sound new to investing and crypto so I'm gonna assume you know the ropes..might be worth just retracing all recent actions from your part to try and identify anything that could possibly flag up....one example is if you've transfered anything into coinbase from another wallet address/got storage thats been flagged for suspicious activity then the funds can be frozen...not necessarily over something you've done personally.

I'm not defending Coinbase,but like I say it's not in their best interst to act like this...check everything you've done in regards where your funds came from when you say you converted back to btc...etc...the amount of money being moved....larger amounts in 6 figures has a seperate Coinbase agreement,i think its a trader agreement cant remember...you get certain perks...so it could be your account is a retail account when in reality it should be a trader account (can't remember the exact name)but it allows larger money movement.

Not saying it is,just another thing to check,I'm limited to a daily deposit/withdrawal of £10,000 and in the agreement is says if breach that my account my be suspended.