--- Disclaimer: Some might feel this answer to be hard to digest. ---
Have you ever thought about what is happening in your kid's brains while they are watching TV?
I mean, from a neuroscientistic perspective?
Basically a fireworks of impressions without any chance to influence what is happening. A child in front of TV will absorb a flood of pictures, sounds etc. in a quick succession. Yes, there is a story and yes, often something to learn, but there is no break, no time to reflect and no chance to influence the speed of the presentation or ask questions.
You might think your children are "zoning out" or "relaxing" in front of TV, but actually they are not. Watching TV is hard work for a child's brain. Of course they will love watching TV, exactly because of the amount of inpressions - a brain loves input. Unfortunately, the filter mechanisms most of us adults have are not developed yet, especially in younger children. Is your child able to get up and walk away while the show is running? Excellent. Most aren't. While the TV is on, they will remain "hypnotized" in front of this box. Hey, I know adults who can't ignore a TV - and the fact that our brain is wired to focus on movement doesn't help, either.
After TV time, the brain will be exhausted (and somewhat "neurally scrambeled" due to the amount of impressions that couldn't be "stored" or absorbed properly) but still in "rapid input mode", while the body will likely be craving exercise. And in my experience that's a recipe for disaster. The pent-up energy has to go somewhere while mental control mechanisms are down due to tiredness. And this combined equals a child in "overdrive" with a really bad mood - or bickering, grumpy brats.