I have no idea why he felt the need to do that cartwheel but I suspect that he was too high in the crease so that when he had to push off to get over to the right he had to cover a lot more distance laterally. He over-pushed and was basically going to slide out of the crease, so he did that cartwheel to try to cover the net before he slid past the net. This is why people hate Lehner. If he had been just a bit deeper in the net he could have just pushed over caught the right post all while staying upright and square to the shooter. Instead he's at the top of the crease trying to cut down angles and put himself way out of position to make the reactionary save.