Check existing paid camapigns. I have seen this type of SPAM bot generated like traffic from Facebook, Google programmatic buys. See if any marketing campaigns launched and they then used to advertise of 3rd party networks.
Look for any tracking IDs that may have been used as part of campaigns. Sadly there is a ton of garbage digital adds served in some adnetworks you just may have been exposed to some.
Also check in case any IT tests around load capacity were performed can sometime generate traffic spikes to.