Why do some people use two hyphens? --
The same people use one occasionally too. They seem to have some rule. Any idea?

A hyphen is a short horizontal line used most often in adjectives like two-seater, as in two-seater aircraft. There is a lesson on hyphens here.

A dash, on the other hand, is a slightly longer horizontal line which can be used to extend a sentence or as parentheses.

In order to differentiate between a hyphen and dash, many PC users use two hyphens to represent a dash -- if you follow my drift.