Correct, no cyprinid actually gets pregnant, at least in the general sense. They do get fat with eggs, however, and this may be incorrectly reffered to as pregnancy.
Cyprinids are avid eaters. They may simply be overfeeding, and if they are of similiar "base" sizes, it is easily fathomed that they may be more hungry or aggresive than the others. Its not really that big of a deal, just fast them for a little while, or feed less rich foods to aid in digestion.
-Lynden