Under the Gameplay options menu you can toggle to Skirmish While Searching before matches:

[![enter image description here][1]][1]

However, in general, skirmishes trigger when:

- The queue time goes over 2 minutes, from personal experience

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.

  [1]: https://i.sstatic.net/GC6MT.jpg