If they are only occasionally flashing against the rocks or substrate, your PH/ammonia/nitrite/nitrate are in acceptable levels, and there are no visible signs of illness; then what you are observing is common behavior among african cichlids, males will often flick/flash as a sign of aggression or willingness to breed.
While overstocking is necessary for africans, there's a fine line between proper overstocking and fish just crammed in everywere
, you may want to thin out your extra males, and aim for a total of 15-18 adults. Bumblebees (
pseudotropheus crabro) should really be kept in at least 125 gallons when in groups as they reach around 7-8" and the males can get very aggressive. They do however, work well when keeping a single specimen in an african community, several on here have done so successfully (myself included).
As for the nervousness, as others pointed out, it could be a sign of water problems, or also changes in or immediate to their environment. I found mine freaked out when I rearranged my living room near the tank, and they hid for a few weeks. Also, a very aggressive male or several males fighting for dominance can make the rest of the fish very nervous and prone to hiding. Close observation can rule that out. Another cause can be not enough territories or hiding places. Behavior can change when the fish start reaching sexual maturity, a need for territories and establishing an adult pecking order can make everyone a little jumpy.
I'd suggest testing your water to ensure correct parameters, then spend a bit of time oberserving the tanks interactions for more signs of disease, or aggression problems.