why did I get a lockout screen? I was just carrying my phone in my pocket...

You got the lockout because the phone's screen was rubbing against your leg or something else in your pocket. This woke up the screen and then subsequent touches were interpreted as passcode entries. This resulted in the wrong passcode being entered multiple times.

If you must keep the phone in your pocket, position it so the screen faces away from your leg.

It may also help if you turn off “Raise to wake” in Settings/Face ID & Passcode.