I don't know. Mark run his own publishing thing. Maybe give him a ring or email. My own theory is that this is also something of a niche coach's book and it has gotten a lot of more general fitness and recreational lifter interest. If there is a delay that might have been unexpected when he figured out demand for his first print run. Which means great success but unfortunately the inability to satisfy the short run demand spike (this happens in business all the time and ironically some businesses have come under serious financial burden or gone bankrupt due to unexpected demand).
That said, I'd just call or email and ask what the deal is. Let us know BTW as I want to add his stuff to my site and I'd like to know if he's out of books for a period.