The most reason why exchange seize user's asset is not being verify account, which is the most policy for many exchange, which everyone has to verify account as know your customer (KYC) before funding.
That's just one of the reasons, but what about all those who did KYC, and then it happens to them? They (CEX) simply do not have to tell you the reason why they blocked you or why they froze your funds. In my opinion, what causes the most problems for users are automated detection systems that have an excessive rate of false detections, and then the people who check these reports have to
"manually" check it all.
In other words, it looks like a fisherman throws a hook into a shoal of fish looking for a specific one, and in most cases he hooks one he doesn't want. CEXs unfortunately catch too many
"fish" that should not be on the hook.