"Order flow toxicity is the measure of a trader's exposure to the risk that counterparties possess private information or other informational advantages."

Usually, flow from other MMs isn't toxic.

Toxic flow can also just be someone who's executing a very large order, even if that counterparty isn't informed. If you fill them as they are starting to work their order, you could get run over as they continue to finish that order and push the price against you.