I'd guess anonymity has a factor in this. It's easier to talk trash when people don't know who you are. It takes away the risk of being shamed for their bad behavior. Not that you can't be identified if someone knows what they're doing, but the average player won't bother.

Otherwise some people are just inherently toxic, and they take advantage of the communication tools in games to spew stupid stuff.

Also, it's just human nature when it comes to competitions. Some people are just bad sports and are not gracious at all.

Best not to dwell on it too much. It's often out of our control. Mute if you can, or have some fun trolling them.