I don't think there's a direct correlation between new HHonors enrollments and people moving away from Marriott. There may be some marginal affect. To make a correlation, someone would need to measure stays or nights, not enrollments.
Like others, I've had a Hilton account for probably 15 years. If my Hilton nights went up and Marriott nights went down, that may signal some correlation. Likewise, if I were a new signup in HHonors but continued to stay at Marriott, what does that say about Marriott?
Increase in enrollments can be attributed to a whole lot of things, often promotions or credit cards. I'm guessing there is a marginal number of Marriott elites that have opened brand new Hilton accounts - certainly not 14M who never had an account there.