How did that happen?

Literally zero idea, I pressed the scroll wheel unlock on my G502 and the sensitivity suddenly changed. I even checked to make sure I didn't accidentally change the profile.

To the best of my knowledge that button is just a physical switch and doesnt have an electrical signal attached to it. 

Have you checked your Logitech Game software to verify the DPI is still the same?

I don't know the exact DPI settings I used but the button assignments were the same so I assume those didn't change.

I know the original G502 has selectable profile's using the G9 button under the scroll stepper lock/unlock, and each profile can have the same button layouts but different DPI's (profile represented by the number of blue lights above the thumb buttons)

That's what I meant by profile previously. The one I use has a specific key modified for push to talk. I went through the DPI options in that profile and none of them felt normal.

Backups, backups, backups. 

I just set my DPI to whatever one full sweep spans all one and a half monitors. Something to keep in mind if you ever need to change it in the future.