do you see the spikes on the graph?
starting around August and going through October,November,December 2017 and happening every now and then quite consistently?
this is when Bitcoin Cash mining is way more profitable than mining bitcoins
so lot of hash power is diverted towards mining BCH,thus creating a sudden drop in the hashrate for bitcoin and longer times between the blocks
cause of this the mempool is slowly clogging up and transactions are becoming more and more expensive
it has been better lately with the bitcoin price not so volatile and bitcoin cash dropping price,but this is a factor to consider still
This has happened a lot less recently due to Bcash changing the difficulty retarget algorithm on 13th November last year. The only times it has happened since then are when the big the ramp-ups in price occurred. It needs a pretty large shift in the BCHBTC price for that to happen. There is always a certain amount of change in the hashrate due to variance of luck.
https://fork.lol/pow/hashrateIf you look at the dotted line for total hashrate of BTC and BCH combined you can see that. Then scroll down to the second chart and you can see that the spikes in Bcash share of the hashrate do not last very long now as the difficulty retarget every block soon makes the profitability very close to 1:1 again.
It all brings back memories of multipool mining shitcoins in 2013 before they worked out decent difficulty retarget algorithms.