Most fun is that this AI is literally closed black box, it is not clear human written algorithm (for more than 5 years already), but set of large mostly self trained neural networks.

In around 30% cases AI system doesn't even provide any reason to Paypal itself, zero, it is just called "suspicious activity".

In another 45% of cases local country operators and managers also can't see reason, it is available to specific group of US managers only. It is made as reason is so fuzzzy and impossible to be proven legally with any proofs.

All other cases are usually related to big spikes of payments, documents issues and such. They are also usually legally fuzzy.

Rumors spreaded by ex developers are that AI system is tuned to provide Paypal enough blocked funds to speculate on risky stock and futures markets (where all this funds go instantly).

Main tip that they provide is to keep activity low and never have above $100 on account balance, such accounts are not interesting and are filtered out by preliminary algorithms to not use more expensive AI networks.