why did bread during the french rev skyrocket?

A succession of poor harvests meant that wheat for making bread was in short supply,and this is what pushed the price up to a level where the peasantry could not afford it.

The Little Ice Age brrought with it a change in climate, producing more colder days and more rainy days, decimating the wheat crop.

they fired it out of cannons?

Or maybe you forgot the word "price"?

Shortages as harvests and distribution failed.