There are three reasons to t-bag in my opinion.
Because you just made a nutty play and are releasing sheer excitement. It's usually accompanied by DPI toggles, fast spinning, and other random button spams.
An intense game going back and forth. Both sides are clutching up and you gotta release that stress from barely surviving their top fragger who is almost omniscient in their gameplay.
You are shitting on noobs and wrecking bitches and need to remind everyone you are the alpha. Doesn't matter that you have a tiny pp because you f**king rock at this game and everyone else sucks.
Reason 1 is fine. Most will realize you earned that excitement.
Reason 2 is similar to shit talk in sports. It's not always classy and some choose to not partake but it ends with the game an no hard feelings should be held but yes it's still irritating to be on the receiving side.
Reason 3 is for asshats who actually think being good at a game makes up for their real life worthlessness. You can't reason with them and they are incapable of self reflection and introspection. The best you can do is continue to speak up because they get their confidence and loudness from others being quiet. I wish companies would take a stronger stance on this.