He is a manchild who is still in the phase of thinking X is the coolest letter of the alphabet. Give it a few weeks and he'll be obsessed with Q or Z instead.
Musk may be erratic in other things, but his preference for the letter X is a constant, just ask x.com (1999), SpaceX (2002), the Tesla Model X (2012) or X Æ A-12 (2020).