It's packet loss (or so I've been told), and it's been happening constantly since before Christmas (for me, and a couple of others as well).
The most frustrating thing in league I've ever had to deal with is clicking to move forward slightly, it happening, and me standing there to watch myself die to the enemy.. Or attacking someone and being unable to pull-back, walking straight into them. I've had lag before, plenty.. a house full of other people on netflix, youtube, and other things all day long, combined with me being on the east coast, I've learned to deal with delay and fluctuations, to time my league playing around it so that my ping doesn't skyrocket..
But flat-out not being able to do anything is just.. unmanageable.
Unfortunately, i don't know of a way we could fix it on our end (not for free anyway).
It's one of those things Rito claims to be fixing, but so far it's only gotten worse for me the past few months.
I'm at the point where i basically just do co-op vs AI (ARAM on a good day), because i know if i randomly can't do anything, my teammates won't be as bad off.
..That or i just don't bother playing, which at the rate it is going, will end up being the permanent solution for me.