Why did many prices drop in 1921?

The answer differs country by country. Vast amounts of wealth was diverted from social goods to war materials during WW1 and were slow to return and retool to manufacture goods. Most of Europe was shattered by the war, cities destroyed millions of skilled workers killed and displaced, knowledge lost. Farming was almost nonexistent in Europe for the same reason. Here in the US we were having the beginnings of a major depression, drought caused by plowing under prairie grasses to plant wheat to feed Europe but that was still a bit down the road in 1921.