Ive been using ios 8 on my ipad air (32gb wifi) since beta 2 and just started encountering this issue on Saturday and last night. My iPad has about 6gb free normally and I think it starts randomly downloading things to go to 0 bytes remaining. After it happened the first time, on Wednesday I plugged my iPad into iFunbox and saw there was 6gb of stuff in the downloads folder. Deleted it all and problem was solved until Friday where it downloaded it all again. I assume it is downloading things because I have a 250 GB monthly data cap on internet (only ISP in my area ). I checked my data usage on my phone and it went up about 6GB. Edit: I checked the Battery Usage and iBooks took up 35% of my battery with "Background Activity" so I assume iBooks may be a potential culprit.