Most of the accounts were probably registered normally with en email address and a handle, only until Twitter presented the accounts with a 'Please verify the account with a phone number', and then they got flagged and banned.

There used to be a loophole in Twitter where you could register accounts en masse (without phone verification), but they have since clamped down on this and now you have to go through rigorous verification steps to prove you're not some bot or trying to run a propaganda campaign.