Much of the time, it is a release of frustration.
I have had a couple of coaches come out and scream at me at the top of their lungs over a call, without crossing the ejection line. And the last sentence is something like, "thanks, I really needed that." 
Many times, it is a stupid coach.
Not much to say about that, other than, High School coaches are the worst of this category. Coaches below the HS level, do it out of ignorance (which is curable).
Sometimes, it is part of the game / show.
A good coach knows when to argue a call to show his team he hasn't given up, and is willing to go the distance to defend his team. You will learn to recognize the difference, by the way the coach approaches you. The coach may even tell you what they did between innings.
They also do it to hopefully alter how you make that call next time, as part of the psychological aspect of the game. Some umpires let a coach get into their head, which alters the way they make a call, just so they don't have to have that coach on their a$$ again. If that happens, and the coach recognizes it, you are done for the season with that coach. They will be on you for every little thing. Only ejecting them multiple times during the season will get them off you in the future, so don't let it happen.