When the Camarilla was formed, the Clan Malkavian was initially passed over as candidates to join, due to their insanity, intractability and the "infestation" of Dementation. Somehow (nobody is really sure how, even today or among the Malkavians) six of the oldest active clan elders managed to suppress Dementation within those that wished to join the Camarilla, replacing it instead with Dominate. Because of this, they were allowed to join.
Centuries later, in 1997 this change was suddenly and inexplicably reversed, referred to by some as the Dementation Plague, by others as the Grand Prank (or perhaps, some whisper, the change to Dominate was the Prank all along). Again, not even the Malkavians know the reason behind this. Or, at least, no explanation can be agreed upon (there are as many theories as there are Malkavians). Just as mystifying, some Malkavians proved "immune" to the Plague and retained their skills in Dominate, while still others manifested both Disciplines strongly.