You say this like it's a bad thing.I think it's a personal thing, some players still see the game as competitive, and they may compete for spotlight.
As a DM I'd far rather see them actively competing for the spotlight than just passively sitting there waiting for it to (maybe) find them; because if they compete the chances are they'll end up sharing it with each other far more of the time than otherwise. And shared spotlight is almost always better than individual spotlight, all other things being equal.
For a whole myriad of possible reasons of which this...Yeah and why does such player get bored?
...is but one.one player gets bored by someone else's spotlight mostly because he is someone who wants the spotlight on himself all the time.
Other reasons could be:
--- the fiction being played out in that scene simply isn't engaging to that player, and that player would rather move on to something else regardless who ends up in the spotlight next (maybe even the same person!)
--- the player - be it in or out of character - thinks the party's current course of action is a dumb idea, and-or that an idea put forward by someone else (could be the bored player, could be a third party or even the DM) has a much better chance of success (this one covers the fight-v-talk scenario in the OP)
--- the player might be finding the humour-seriousness level off-kilter e.g. the spotlight's on someone playing pranks and the player wants to get on with the serious storytelling; or the reverse.
These sound like my sort of players!They're still our friends though, so we just learned that when they are around we have to accept that if we play a game with them we should not even bother playing seriously that night.
I've played in games with people like these; often my evening would be largely spent doing nothing except roar with laughter at the ridiculous things they'd do and the even-more-ridiculous arguments they'd get into.
Best entertainment anywhere!