The only time I've ever noticed a 'plateau' similar to that before was when the network was down. I don't think that's what's happening.
Lookit this.
theres a new kid on the block
Upbit
and he's KICKING ASS
83% of the volume!
over five times that of Bittrex?
(unless I got my math wrong...math is hard)
The other exchanges are insignificant.