The priorities that I
tend to exercise are that aircraft operating flights have priority over aircraft under tow who have priority over vehicles.
However, anything can sway these decisions and the number one priority is to keep the aerodrome moving. Examples may be tight slot times or other flow restrictions, an aircraft running particularly late, or the best use of the geography available. Sometimes an aircraft that is ready in a particularly awkward place has to push at some point and will delay somebody - you may just be unluckly that it's you! Finally, sometimes an event (e.g. push and disconnect) just takes longer than we might expect normally.
Very inconsistent operation
Give 5 ATCOs a problem and you'll get given at least 6 solutions... Everyone has their own way. There is almost certainly a reason for every decision, remember you don't have all the information to hand. And if that fails to convince you - final straw is that we're only human.