Not sure why I even phrased this as a question. The obvious answer is the girl. The issue started when she agreed to this trust fall. The second issue is she is probably drunk. The third issue was how poorly she jumped. Where are you going?! To the right?! Do you know how trust falls work?!?
She went full QWOP:
Gotta love how she started petting the guy she fell directly on:
Sorry that I fell directly on your neck. Let me give you some motherly attention.