My understanding of POS is also limited but I think the amount of stake depends on "age", so if the wallet was sitting for a few days idle and then started staking it will "catch up" so to speak. That part is likely normal, the crazy APY is not.
I think you're right. The wallets were down for a few days cause of the PayBase hack, right? That spike is around when they flipped the switch back on so then staking would automatically catch up.
That's what you want, otherwise people who had their coins in there staking would not get their interest for those down days.