User blog comment:TheTobarMethod/Why is Mid one-sided?/@comment-216.36.26.248-20130315111348

Any of the three lanes can produce this effect. The reason you notice it with mid in particular is because it happens to be populated heavily by characters with "Assassin" in their descriptors. Additionally, by virtue of being solo, if a small advantage is gained and pressed correctly, there is no easy recovery.

Returning to the Assassin part, it also is particularly hard for junglers to assist mid lanes. A jungler needs time to respond to an ally in duress. In the bruiser-heavy top lane, plenty of time is afforded by skill sets and item builds. In the bottom lane, there is a support who typically gets very practiced in stalling deaths (and many ADC these days have escapes and some crowd control too). In the mid lane, you can be killed in a volley of spells and your opponent can be already leaving by the time the jungler is able to be on scene.

Lastly, the mid lane will typically have as many or more minion kills than your ADC. On top of that, the mid lane will typically have more exp than your bottom lane and a character with a skill set specialized in killing weaker opponents. This makes it both a good idea and easier for a winning mid lane to go bottom and wreck the ADC as well -- further gaining a lead and slowing the other team down.

This is how league of legends and every other dota-like I can think of is designed. Small advantages, when pressed, become large advantages. Overcoming a deficit requires equal parts luck and skill.

tl;dr: Assassin mid lanes with lots of money do what you'd expect Assassins to do.