The aircraft was a 737-900ER - known for balance issues that can cause it to tip on it's tail. They would have been keeping you seated until they had done something to avoid that occurring (unloaded some bags/cargo, or more likely put a tail stand in place). The jetbridge wouldn't have been put into position until after that had been done.

"Tail Tipping" on UA aircraft -- why, prevention, experiences, .... for more details.