It must have been called a DQ because of Dirrell's reaction. There's literally dozens of examples of fighters getting punched on the floor with no action taken or at most a point deduction.

In recent times...

Berto-Rodriguez (ironically the ref was Laurence Cole, he did nothing)
Barrera-Marquez
Oscar-Mayorga
Naz-Kelly
Haye-Barrett

This would be endless.