has it happened before?
Bid-bots downvoting? No. Most of the Steempower is delegated and downvotes are a net-minus, meaning if they'd downvote, they would have to pay delegators out of their own pocket. With "25% free-downvotes" however, the risk is of course there. But again, who would keep their delegation on a bid-bot or even use it, that is downvoting out of revenge.
It could also be mitigated later by making a SP threshold for downvotes.
Example:
You might want to think of things like this to prevent flag wars. It has happened and can in the future... Especially if someone like Facebook buys a huge stake in Steem while prices are low to censor.