This happens when ripe grain is not harvested, especially in wet weather, when the corn is laid or flattened by wind and rain. The damp conditions encourage the seeds to sprout, which is the natural thing for them to do!

http://www.ppdl.purdue.edu/ppdl/weeklypics/12-12-05.html

It also happens with wheat and other grain crops.

This is one reason why harvested crops must be dried. Another is to prevent moulds or fungi and other pests growing on them.