Downshifted by itself.

[gyazbek]

Hello! Today as I was driving back from school, I accelerated to merge into traffic, shifted into second, third, and then as I am about to take a turn the bike downshifts to second causing it to over rev and the rear wheel to skid. Would there be any reason why the bike might have down shifted? (I was ready to shift into 4th before the turn, and that scarred me because my foot was positioned under the gear lever, so I didn't push it down.)

Thanks ahead of time!