[Anotheroneagain]

500mm isn't low by any means, it's moderate. Almost all Europe has around 500mm annual rainfall. It may seem low compared to let's say Mississippi, but that is because Mississippi is as wet as it gets outside the tropics.

The fields were abandoned because the prices were so low that cultivating them wasn't profitable. This in fact could have caused the drought as well, as much of the rain in such climates is recycled - the water that evaporated rains back down. And, it can't evaporate well without plants to bring it up from the soil, but only the surface dries out. This might have lead to the dried out top layes getting repeatedly blown away, until the fields got successfully overgrown, and the rains returned.