“Why are my ears ringing after wearing earplugs?”

With earplugs moisture can build up in the ear canal and with moisture you get inflammation or increase blood flow that could cause ringing in the ear. That should dissipate quickly or another way to treat it is with some grain alcohol putting a few drops in the ear canal which will evaporate any moisture.

Ear plugs are most likely damping ambient noise while in ears. When removed the increased sound is most likely stimulating damaged hair cells in outer cochlea