If you care to look closely (the series is over, so no problem if you don't) PLD's leg comes through from behind and makes contact with Drai's inside leg as he leans into the contact. That's what puts him off balance, and the Dubois pushes from the top to put Drai to the ice. I couldn't tell for sure from the angle, but it looked like the butt-end probably made contact.
If one really wanted to get technical, it could be considered a slew foot (leg sweeps from behind with opposing force on the upper body), but not the most egregious. But either way, you're not allowed to do it. And combined with an apparent butt-end, this probably isn't the penalty to complain about.