It’s simple. Irregular Activity in this case is being defined as things like connecting from multiple IP addresses in a short amount of time. One of our staff members has an issue with the Xbox ONE which causes all tiles to go dead if there is any sort of internet disconnect at all. In order to fix this in a reasonable amount of time and force the Xbox ONE to re-connect the tiles, the person must reset their router AND their modem.
If you’re using cable internet, every modem reset grants you a completely new IP address. If you are connecting to PSN from a new IP address all the time then Sony has reasonable cause to label your account as having irregular activity and reset your password just in case. Even if you don’t have an Xbox ONE, most of us have cable internet. And a lot of people have been resetting to try and fix their NAT connections as well.
There’s a huge chance that you’re getting your IP reset more often than you think due to connectivity issues and this is largely why your PSN accounts are being reset by Sony. Hopefully soon the PSN will calm down a little bit on this practice and the Xbox ONE will fix its tiles so people don’t have to reset their modems so often.