Every work place is different, with its own customs and ideas about how things should be run. Some workplaces are super casual, while others value a strictly professional environment. But most are somewhere in between.
When you spend 40 hours per week with the same colleagues, your relationships expand over time. You will probably get to know all about Kathryn's favorite TV series or Jonathan's weekend trips with the kids. Since you're at work, you have to be focused on your job for the most part, and having happy hours can allow people to relax and let loose. Without the confines of a cubicle, it's way easier to get to know others in your establishment.
This manager has been going to happy hours with their colleagues for some time. And their most recent one was special---they were celebrating a colleague who was moving on to another job after spending years at the company. For the OP and those at the happy hour, it was a pleasant event.
But u/back_to_eden's boss took this impromptu event personally. She has apparently been trying to get every employee to dish out the details of what happened that night. At least commenters gave the OP helpful advice.
After you check out the story below, read about this employee who spited her boss by giving herself the funniest/worst haircut you can imagine.