I watched Scott Manley’s video analyzing the launch and he said that the booster experienced a sudden acceleration backward when the second stage ignited and pushed against it with more thrust than the booster was generating, which would have caused the liquids inside it to surge up and then drop back down again. Seems like that would produce a stronger hammer effect than the booster was originally designed for.

I could, of course, be misinterpreting the video. Or Scott could be wrong. Eventually SpaceX will tell us what really happened.