Well, my best guess for the bike taking out half the town is probably the amount of gas you had in it, the size of the explosion depends on the amount of gas, and they REALLY boosted storage capacity in some recent updates without changing the explosion system… yeah…
Now as for the explosion in and of itself, hitting things tends to damage parts of your vehicle, shrubbery, creatures, walls, etcetra. Probably hit one of them without realizing it.