But I did not understand why the hell he collapsed on the court like a dead fish and showed everyone his tongue.

This is a stupid gesture, I do not approve of it.
Although the victory was certainly a big grief for the Djokovic.
It is goal celebration from FIFA game. There was lots of people who did not understand the gesture but there were also some who were thrilled by it.
Personally, I find i a bit strange, but I understand the idea and intentions behind it, so well done. Tennis is a sport that can use all the extra media attention and even few bad boys. It has been too long from last Kyrgios incident