Here's a common question that's asked, and there is a simple explanation. First of all I want to exclude regions that have many hummingbirds for this reason - When the population is much bigger, it becomes impossible for one bird to chase away all other hummingbirds. They still remain cranky and continue to squeak out a warning, but when there are multiple birds all doing the same thing, they just try and work their way into a feeding opportunity.
For the rest of the population that has far fewer birds, something different occurs. Hummingbirds are seriously possessive of their food. Once they've fed from a feeder, and in their own mind determined it's now theirs, they start to protect that source. If they catch another bird feeding at their food source, they quickly fight to protect it. As the number of birds increase you sometimes get situations where a few birds start believing they each own that feeder, and so they fight and chase each other around the yard. Then you get the situation where certain birds are pushed out of feeders. Each feeder has an owner already, and the rest have to sneak in for feeding opportunities. Occasionally you'll get a sneaker that finds an open feeder while the owner is away or sleeping on the job. That sneaker isn't in aggression mode. It just wants to grab a feed as quickly as it can without getting noticed. Finally, you get a second sneaker that arrives at a feeder with another sneaker already there. Neither wants any trouble, and they are just hoping the other won't be too upset with sharing. Neither one is in aggression mode, because both of them each know they are the trespasser and neither one owns that feeder. Sometimes you get one peaking around at the other just to make sure they're in agreement, but then it's back to feeding.
There is a pecking order within the hummingbird world. Some birds just know they are at the top. Ziggy(my dominant male) for example, he doesn't question whether he can take on another bird, he knows it. Over time, and several combats later, the toughest will take over, and the others simply understand that. If sharing means they get in a feed, then they'll accept that. When the young start to discover and practice scare tactics, they will fly up into the face of another, and flare the tail and wings to try and intimidate. Sometimes it's effective and they realize the power they contain, while other times they realize it was a really big mistake while being chased around the garden a dozen times.
Two non-confrontational Ruby-throat hummingbirds just wanting a drink N.E. of Edmonton, Alberta, Canada. Mid-August 2018