I had the same thing happen twice. First time, the Etrex was near a leaky bottle of DEET, and I think the fumes make a pretty good glue solvent. That one went back to EMS and they swapped it for new.
Happened again, this time I think it was just the heat. I pulled everything apart and cleaned it up. Then put on a strip of double-sided indoor/outdoor carpet tape, trimed to fit with an exacto and wrapped the bumper back on. Been holding up very nicely all summer.