Your favourite competitions matches can be sorted either by priority, the default setting, country or A-Z. Note that this only affects matches which are grouped under the "Competitions" header.
To change sort order, tap the "Competitions" header in the Calendar, or go to Settings -> Following -> Competitions. In the "Competitions" screen, tap the "Edit" button in the upper right corner. Now you can either drag-and-drop your competitions into the order you want them, or use the "Sort" toggle in the upper left corner.