The guy at the plate was a lefty pull hitter. They were playing everybody in the infield pulled over to the batters right to try to prevent a hit.
Don't know if the coaches somehow forget to tell the 3B to okay a little closer to the line, or if he brain farted all on his own. He definitely should have gone over to hold Elly on street he got there, but in his defense, nobody tries to steal home in that kind of situation at the major league level.