Throughout the game, the Oilers have a tendency to - and only the players and coaches know which it really is - get either too nervous, or to casual. You can see it in the powerplay. When McDavid is telegraphing his passes, and just puts the puck right on to a Hawks stick. When Draisaitl can't handle a routine entry pass from RNH, bobbles it.
In the last minute, McDavid with speed up the wing, he drops it without looking to a guy outside the zone. And it's not just those guys, it's all over the lineup. Nurse icing the puck multiple times for no reason. AA going 'controller disconnected' after getting a great neutral zone pass from Drai.
People want to talk about one guy making one bad play. But it only gets to that point because of numerous unforced errors. It was this way last season, it was this way the last time the Oilers played in the playoffs, and it will be this way until the team learns to - as Mike Babcock used to put it - "leave no doubt".