Eastern Orthodoxy - When a liturgical hymn to a Saint is composed (such as a Troparion), how is the mode/tone for the hymn chosen?

Canons, troparia, kontakia, aposticha, stichera, etc. all have a tone/mode associated with them. How does the author of the hymn choose which mode/tone to use? Is it the same for all, or is it a little different for each type of hymn? (I'm referring here specifically to hymns written to commemorate Saints. Hymns composed for the Octoechos or for Pascha I already somewhat understand why they are written for a particular tone/mode).