yea, may be they are upgrading their S9 farm to S17
No "maybe" about it, they have a video on their facebook profile of them upgrading all the racks and removing the S9's to replace them with S15/T15/S17/S11 style form factors.
Where do you think all the s9's came from that have gone for sale on their site?
https://www.facebook.com/Bitmain/videos/2566497033390022/Most likely the cause in the jump in hash rate this week as well.