It’s a force play because R1 has to retouch his original base (first) after the ball is caught. He is “forced” to go back there after the catch is made. So he doesn’t have to be tagged.
The actual wording is:
7.08 Any runner is out when—
He fails to retouch his base after a fair or foul ball is legally caught before he, or his base, is tagged by a fielder.
