Echelons are diagonal lines of riders that stretch across the road. They form when there’s a crosswind because the place to shelter from the breeze is no longer behind the rider in front, but behind and to the side of him.
Because of their form, each one can only accommodate so many riders. You can see in this image how the riders at the back of an echelon have nowhere to hide. Exposed to the wind, they have to do just as much work as the guy at the front and if that rider sets a fast enough pace, they can’t keep up and the group will split. Without drafting, this can very easily happen. Drafting is all that binds the peloton together.
When the peloton splits, a new echelon will form. Depending on the wind and the road, this can happen again and again until you reach a point where there isn’t a peloton and the race looks like this – lots of little echelons, all being driven along at different speeds. Even if the wind drops or changes direction, the splits have already been created and you’d better hope you’re in a group with strong riders because it might well be every group for itself until the finish.