Don't think the system is so simple to operate only around you. It probably noticed you're the fourth or twentieth person who slotted into an overpopulated role. Rather than arbitrarily waste time it already knows it's gonna have to autofill someone. It doesn't have to let you wait to figure that out. Because if it's working with 20 mids getting a certain amount more a minute and has almost no supports getting not as many a minute it's obvious what it has to do. The matchmaker has to do this at every elo bracket while generating balanced matches and teams so if you think about it it's a pretty complicated system.
http://nexus.leagueoflegends.com/2017/06/ask-riot-autofilled/
I found that for you. It's pretty recent and has some inklings of whats going on behind the scenes. For one it keeps score on how much you autofill recently. The longer you go avoiding autofill the higher the odds it will autofill you soon. When you do it goes back down. I've noticed this too and can game it by selecting fill secondary and get a role like jungle or adc (I can play them all) if I don't feel like supporting and get autofill protection. Some of the stuff they do is probably overly protective in function of those that select fill or support occasionally but it makes a lot more sense than "if you select fill you might as well be selecting support" 100% of the time for someone that just wants to be a jungle primary support secondary main for example. You couldn't do that when this initially released cause you'd be support actually every single game. Then that guy would have to drop support to play jungle and learn another role worsening the problem.