It's not about miners, it's about the increase in the number of transactions. I don't know if it's some sort of spam attack or a mark of Bitcoin success.
There are days they are much more than the current architecture can handle.
Top size of mempool in the last 2-3 days was 44 MB, now it's some 33 MB.
https://btc.com/stats/unconfirmed-txI have high hopes that in a couple of weeks, when SegWit will be actually in use, this will not happen anymore (for long time, hopefully).
SegWit will not even work unless 95% of the miner hash implements it.
One mining pool with 8% hash has stated they will block SeWit, and want BTC unlimited instead.