Nothing. It fluctuates all day depending on what coin is being mined.

Coin switch.  Different coins result in different reject rates because of diff/speed of the coin. Clevermining pool is more aggressive in pursuing fast coins that have good profitability despite higher reject rates so you'll see more varied reject rates here. Others like Waffle avoid these fast coins so you won't see them have high reject rates. It's more than network latency, server issues, and miner configuration.  People tend to forget about the fast coin aspect here.  Oh well.