I read an article a few months ago that introduced the idea that throwing a baseball above 107 mph is relatively impossible based on our physical limitations. It seems to me that notion is going to be challenged pretty soon.
I myself played against a pitcher in high school who was throwing 93 mph so I can only image the difficulty of hitting a 105 mph fastball. I was happy just to make contact and hit a weak foul ball down the first base line haha.
I attribute the increase in pitching speed to the new dedication baseball players have with their physical training. I currently work in the Sports Performance industry and spent time this off season training MLB players. In this day and age all athletes regardless of sport or position take their strength and conditioning seriously as they know it is now a requirement to compete at the highest levels.