1) Large institutional long investments in a certain type of wheat future (Chicago soft red winter) started crowding out the real customers of physical wheat, such as bakers.
2) Because wheat varieties are moderately fungible/exchangeable, the downstream bakers started to use a different brand of wheat (Minneapolis hard red spring) and changed their recipes accordingly.
3) This second class of wheat is very widely consumed, and bidding up its price caused riots and food shortages around the world (or so the reporter states. He didn't provide specific examples, as this predates the Arab Spring).
The overall effect is if a particular food staple suddenly became a fashion item, with wealthy people buying it off the market for non-dietary purposes at such a rate that poor people could not afford it.
However, the key bit I don't understand, and which the reporter doesn't explain, is how commodity fund investors didn't lose their shirts by betting on wheat to keep going up. If they did lose their shirts then this bubble is unlikely to repeat in the near future.