The Neurocritic reviews evidence for the whopping increase in drug brand names beginning with the letters z and x starting in 1986 and quotes the conclusion of the study's authors:
Reflecting their infrequent occurrence in English words, x and z count for 8 and 10 points in Scrabble, the highest values (along with j and q) in the game. So names that contain them are likely to seem special and be memorable. “If you meet them in running text, they stand out,” is the way one industry insider explained. Generally, they are also easy to pronounce.
The last point about being easy to pronounce is basically nonesense, so forgive them that, but their basic point that infrequent sounds are more memorable is basically a restatement of Zipf's Law and may have some truth to it.
I can tell you this, there are entire companies that charge high fees to help manufacturers develop brand names (see here for a discussion of what brand name developers do). I worked at one of them ever so briefly and I found there to be a mix of legitimate linguistics and voodoo linguistics mixed together in the "research" they prepared for their customers. I also found a resistance to serious linguistics for two reasons: 1) the customers didn't like science (I'm not joking; this was a serious obstacle) and 2) serious linguistics took too long and didn't come to firm conclusions. Typically, we were asked to initiate, perform, and complete linguistic research on brand names in a matter of weeks.
Ultimately, though, it was my conclusion that a product's name simply was not that crucial to its success, which teetered on the manufacturers overall marketing strategy more than the name. Think about Google vs. Microsoft. So, the rise in z and x named drug products is a fad based more in the board room than in the marketplace.