It looks like another round of copyright strikes is going out over in Youtube land, with reaction channels taking the brunt of the attacks this time around. Quite a few in the reaction community have taken their videos offline to avoid marks on their accounts.
The claims are apparently being sent out by Discovery Family, with season 5 and Equestria Girls episodes taking the brunt. Chances are it's another update on Content ID causing the sudden wave, considering both of those are currently up on their official channels for purchase.
For now, if you or someone you know is using straight footage from episodes season 5, it might be a good idea to temporarily hide your videos until it blows over. There is a good chance this one will stick though. While Hasbro has been alright with posting clips and simply putting ads on them, full episodes on Youtube, regardless of reactions, tend to be nailed, unless they minimize footage or break it up enough to not trigger it.
Thanks to Yon for the heads up! A random list of youtubers effected have been listed below.
IMPORTANT UPDATE! PLEASE WATCH! by DemonAngel 13
Reactors are in danger, please spread the word! by BronyBurningAxe
All MLP Reactions Have Become Private. by DYWTBA_Universe
Channel okay, but content hidden for now (copyright garbage) by Applegeek
Seasons 4-6 Are Temporarily Private by PieDislikerSwag
Where's The Season 5 & 6 Episodes At? (Check The Description) by Bronymonster44
Update Possible Copyright Problems by PotterBrony82
Discovery Family has gone flaghappy [All S5-S6 mlp private for now] by The Silicon Brony