1. There are simply more videos of heinous things because cameras have become ubiquitous and most people have their cell phone glued to their hand at the ready. I remember when a camera on a cellphone was rare enough that they actually called them "camera phones". Police probably have always conducted themselves as they are in these videos, you're just seeing it now.
2. You don't usually see the entire interaction and also cannot use your other senses in the environment. If there is the smell of drugs for instance emanating from a vehicle or house, it gives police exigent circumstances which may come across as an overstepping of bounds to viewers.
3. The individual in question may be a known dangerous person and while they are reacting calmly in the video, the cops don't want to take a chance of being a victim.
That all being said, there are many instances where LEOs have clearly gone too far despite any of the points above. Too many people resist making the situation worse because they are "within their rights". I am not condoning bullying however, you wouldn't push a bear in the wild but instead give it the respect it's raw power deserves. Stop fucking with the police, record everything and do the equivalent of curling up into a ball when a bear attacks - be quiet, say yes sir and show you pose no threat. It clearly won't always work but your odds of survival are much higher.
As far as the cops running around in military gear, it's completely unnecessary bullshit. A lot of officers I've met clearly have a complex and are on a power trip. They wanted to join the military but didn't for whatever reason but now they can pretend to be part of the Bad Ass club. Many people thought Malcolm X was extreme in his views on blacks protecting themselves and striking back however, I think he had a valid point. If the federal government doesn't make some laws regulating and severely scaling back the power of law enforcement soon, people are going to have to go guerrilla in calculated ways.