They didn’t say what that criteria was

They have since clarified that it was an issue with the tower, not the booster. Although "Tower is go for catch", was called out during the webcast, it seems like a later check identified an issue which triggered the abort:

Following a nominal ascent and stage separation, the booster successfully transitioned to its boostback burn to begin the return to launch site. During this phase, automated health checks of critical hardware on the launch and catch tower triggered an abort of the catch attempt. The booster then executed a pre-planned divert maneuver, performing a landing burn and soft splashdown in the Gulf of Mexico.

I hope we get to find out what the "automated health checks" were.