This happens when a mob uses a /shield disc on the mob you are hitting.
What happens is - every hit you make against your target is halved in size, and the other half becomes a hit against the mob that shields your target.
So instead of one hit for 100 damage, you see 2 hits, one hit against each mob, for 50 damage.
In this case, you were seeing a triple frenzy, split into 6 hits against 2 mobs (who probably had the same name anyway)
Frenzy does 1 to 3 hits, with your skill level increasing the chance for the greater number of hits.
For the first several levels, frenzy does good damage - later it gets quite ordinary, then you get loads of AAs to bring it back up to reasonable.