Such, at least, is the hypothesis put forward in a Southern California newspaper article, linked above. In fairness, the article gives no evidence of being based upon an extensive survey of the the churches affected. Looks more like a couple of telephone calls, a brief one to a local MCC pastor, and a longer one to the founder and Archbishop of a micro-denomination called the Ecumenical Catholic Church.
And these calls turn up one particular oddity.
The ECC bishop, Mark Shirilau, is pictured in his mitre and carrying a crozier, but says that he actually worships in an Episcopal parish. This is just as well, since the ECC has declined from its 1995 high of 2000 members to 500, spread over 20 parishes, many of which gather infrequently. The MCC pastor, the Rev. Nori Kieran-Meredith, says that "she is a practicing Catholic."
We're not sure what to make of this. Perhaps the reporter didn't get the quotes right, or know enough to follow up on these curious remarks. But it easy to see why small start-up religious movements are going to suffer when their leaders actually prefer to worship somewhere else.