Margin-call selling. Even today's modest sell-off is probably putting the squeeze on over-leveraged weak hands, and their typical reaction is to sell their best stocks and keep their worst ones. Then the avalanche continues.
While waiting and waiting for the Uranium price to go up, trading URZ as well as URG is the way to go. IMO Both stocks will move again when a date is set for any kind of restart in Japan followed by a pullback. I dont think a Japanese restart will affect the Uranium price much in the short term due to their stockpiles.