It can be triggered for opaque reasons. My account dates to February 2007. I was prompted for a phone number a few years and given no other options to recover the account. Burner & VOIP numbers that work for many other things, including SMS verifications, were rejected.

I suspect the reason was some rapid changes in my IP address in a short period, together with a lot of Twitter tabs open – whose constant background requests often seem to trigger, for me, some sort of Twitter-side connection-slowing. (Their own shoddy, high-weight design makes my normal usage pattern look like a DoS attack to them.)

So your style of usage, moreso than your account age, is likely for being spared their arbitrary phone-number inquisition.