well, gyms cause a lot of frustration because of bad design choices by the developers. 10 slot limitaion is only one of many flaws...

when you are on the underdog team in your area and you finally manage to find another area with permanent, stagnant gyms from your own team, they're most likely level 10 so you can't put anything into them. and since it's very easy to open up one spot by defeating a single pokemon, yes, players are using this to make room for themself. especially high level players who don't have much to accomplish anymore on their main accounts and also don't have access to local gyms in their own area.

having an alt on a different team is very beneficial for this because people usually don't have time to travel across the city or to the next one very often just to place their defenders into some gyms. so they need to make their journey count.

bringing a second account (or a friend from another team to cooperate by making space for each other) is almost mandatory.

i wouldn't even blame anyone doing this honestly. niantic needs to rework gyms entirely. they should really offer different ways to acquire daily coins AND dust and just come up with a new system for area control. the current one is bad for all teams including the dominant one in a respective area because of cannibalism.