|Ain't no party like a squid party.|
To be fair they might also be congratulating you on a great play. You can't tell unless you take the time to look at people's actions and get to know the culture of "quiet" games like Splatoon and Rocket League. Both games are heavily team driven and community focused, but both downplay voice chat.
Instead, you gain little clues about how people use the small text prompts available to them or how they communicate using the game's standard actions. Sometimes this means you witness an impromptu cease fire and have a squid party where everyone flops around without shooting each other. Sometimes it means that someone is flopping on your corpse or, as the poets call it, "squidbagging."
There's more opportunity for chatting in Rocket League, but there's also more opportunity for trolling. There's a hefty current of sarcasm in many games. Players will say "Nice shot!" to shots that are decidedly not nice. They'll "thank" players who make mistakes or shame teammates who blow saves. One of the most fascinating (and infuriating) bits of Rocket League culture is whether or not you skip replays. I'll say it right now: if you try to skip my replay but never skip yours, you can go straight to hell.
That's the funny thing about these communities: no one really talks about the norms in-game. They're transferred by osmosis and turned into doctrine on the message boards. It's kind of like the vast list of unwritten rules of baseball. Don't stand at the plate and watch your home run. If your guy gets hit, you have to hit one of theirs. They aren't officially enshrined anywhere, they're just cultural norms that have slowly built up within a game.