Wheat sprouts revert to seed under the following conditions:

• adjacent to planted blocks at light level 15 (which in turn must be less than 4 blocks from a water block)
• seeded blocks at light level 14 or lower, for example in the shade of a tree (seeded blocks must be less than 4 blocks from a water block)
• Wait until nightfall (time set night)
• Approach seeded blocks and the seeds will jump out.

This does NOT occur at 14w20b!
Picture shows jumping seeds from blocks at daylight level 14 (standing on 13).