Why do players swap positions?

Let's say, for example, you have your CF set to swap with you're RM. What makes him swap?

I assume he would swap if he isn't playing well against the defender?

Why do players swap positions (other than the obvious that you set him to do so)?

If the opposition is concerned about your star man, they are likely to man mark him. Therefore, if he swaps positions with another member of yoru team, it is likely to send the opposition stratergy into melt down and leave gaps for your team to exploit.

As for how and when the game/ME decides the two players 'decide' to swap, i am not sure.

I would imagine it's purely random

I think it's random too. I always have my two strikers swapping positions, although that's only because I can never decide which side to play my two strikers.

Its more a freedom of position thing, say you have it set to a right winger and striker. If the striker gets the ball and sees a lot of space to the right he might run there at which point the winger will swap over to provide the option in the middle so its really a fluid thing that happens all the time.

I can't say definitively, but I think Quackje has the right of it. I mean, from back in my youth days, playing rec here in the U.S., we'd yell "Switch" when, for example, the right fullback overlapped past the right winger and wanted the winger to cover his defensive responsibilities.

From what I've seen of players swapping in the ME, it looks like its much the same: my strikers swap when, say, one drops a bit deep and dribbles around the arc, and the other one makes a run into the space the first's marker had vacated.

The only thing that strikes me as odd is that they are then swapped until the situation arises again, rather than attempting to return to their "default" positions when given the opportunity via dead-ball. That's what I'd want for my strikers, or for the DR/MR swap, but I can also see swaps IRL (such as C.Ronaldo playing either wing for Man U) where the swap clearly takes place for five, even ten minutes before swapping back.

Interesting answers. Thanks.