[dotnet00]

You can see that sometimes in the landing footage, the legs are pretty flexible, and they get some help from RCS thrusters at the top of the booster. Plus, as soon as the engine has purged any excess lox/fuel, a robot slides underneath and tries to latch onto several points, which probably helps by bringing the center of mass down.

There's footage online of some of the early landings when the robot had not been implemented, where boosters slid across the barge in part due to landing at a bad angle.