Once one person leaves a party or event, others feel it is now "safe" to do so. Often causes a significant exodus from a gathering soon after the first person departs.
After the English TV show. When the character Bagpuss goes to sleep, all other characters go to sleep.
As soon as you left the party it shut down, seems you enacted the Bagpuss Syndrome.
I felt guilty that I wanted to leave, but once you started the Bagpuss Syndrome, I was fine with it.