fish

It's called "flashing", it's not a good sign...
http://www.koivet.com/html/glossary/glossary_details.php?glossary_id=1&category=Symptoms

most of the time it's some kind of parasite. Although some types of fish rub on things before mating. What are your parameters? Also have you had any sick fish lately, are all your fish eating ok?

Flashing is a sign of a parasite. You can use a commercial treatment if a parasite is suspected of being in your tank. Flashing is also a sign of poor water conditions.

Flashing can also be a sign of ich. Do you see white grains on the fish? If its ich, try the salt/heat method before going out any buying chemicals that may not work anyway.

my guppies are always flashing for no reason.

My largest barb flashes against the roots & branches in my tank (seemingly) when displaying aggression.

None of the others do it and she's been doing it for years, I know there's nothing wrong with my water parameters and they do not have ich.

It could be mating behavior or dominance behavior, I have no idea, but it is not always a sign of parasites.

If ALL of your fish display flashing at once then I would say you definitely have some type of parasite issue. Fish flash to "scratch" against things when parasites are making them uncomfortable (after all they have no hands to reach the itch).