I've seen Cavendish do that very same move that Keiser did.
Notice how he looks back, then casually forward as he eases downtrack squeezing the guy. Cav did the same thing at the Rio Olympics and actually caused the guy to wreck.
They are expoliting the idea that they are not responsible for what happens behind them if they are doing a legal move. "Oooopsie! I didn't know anyone was there!"
And, yes, Keiser's move seemed to be legal as he was the lead rider and the sprint lane was not occupied (beside or ahead of him). A trailing rider cannot dive inside into the sprint lane and pass. They must pass on the outside.
Basically, it's a veteran trackie dick move.
(turn your volume down. loud music to avoid copyright issues)