Rear tire tearing away from bead
Underinflation is bad for tyres.  Your pannier won't have caused this, other than adding more weight to an underinflated tyre.

Its likely that riding underinflated, combined with stresses of changing the tube last month, have damaged the tyre.  Possibly a tyre lever used wrong may have contribited too.   

You should replace this tyre, and inspect the front tyre for similar damage.



I see a moderately knobbly tyre - if you're riding roads then consider more of a neutral commuter tyre tread for easier riding.