There is a seasonal variation with mining, as it gets very hot in the back bedroom with the sun full on in the middle of the summer (where I'm sitting, lol)
And what exactly are you comparing the Dash X11 hash rate to?
All scrypt and sha256 coins are 100% ASIC now, meaning the hash rate will keep jumping up every few months with the next generation of chip design, they should not be on the same chart, unless you account for the asic multipler factor somehow by scaling down.
Only compare to other X11 coins, and that will give a real comparision, suggest comparing to startcoin, cannabiscoin and digitalcoin as they have good volumes, there are not many others of high volumes really, if you want to analyse the network from a trading angle I suggest you look at the node charts for each coin, that is more telling as it tells how many hot wallets are and also the seed nodes that enable blockchain downloading..
It is most likely, many small miners have hung up their rigs and set up a few masternodes instead being far easier and less competitive to do.
Btw, startcoin is on a 24 hour cycle for hash rate swing, and Dash is on about 22 hours, funny eh, and they phase between each other, but have nearly same hash rates, almost like they are a binary hash system, lol