The pilot reported that, after an agricultural application flight, during the landing roll in a rain shower and with a crosswind, the tailwheel-equipped airplane hit a "bump" and became airborne. He added that the airplane was slow, drifted left, and touched down hard. Subsequently, the airplane exited the runway and ground looped. The pilot reported that he should have landed farther down the runway to avoid the bumps that caused the airplane to become airborne. The airplane sustained substantial damage to the fuselage, empennage, and left wing. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. An automated weather observation system located about 19 nautical miles from the accident site reported that, about the time of the accident, the wind was from 260° at 12 knots. The pilot landed the airplane on runway 18.