Sometimes when I'm in the overwatch queue I get placed in a Skirmish match while I'm waiting. But sometimes I dont, even if I'm on the queue for a while.
What determines if I get into a skirmish match? Are there some options I can set?
Under the Gameplay options menu you can toggle to Skirmish While Searching before matches:
However, in general, skirmishes trigger when:
If a game is found before then, and earlier on, you will not be placed in a Skirmish and instead be placed in a match right away.
There is a a setting called "Skirmish While Searching" under the gameplay tag, but this is turned off by default. If you wish to turn it on, it will then put you into a skirmish match if it taking awhile to put you into a match. Otherwise, when you get put into a skirmish with this turned off, it is usually because you got placed into a match that isn't quite full yet and is yet to start, or someone just left before the start and it puts the game into skirmish mode until it finds a new player to fill the spot or times out and reinstances the game.