I bet one common reason people leave the mormon church is that they actually get saved.
Yes.
And the sample of John Dehlin's study shows this to be the case for 11% of those Lds who self-label "disbelievers."
Per page 7 of this link -- UNDERSTANDING MORMON DISBELIEF: Why do some Mormons lose their testimony, and what happens when they do? -- 1 out of 11 Lds disbelievers have moved on to a Christian (non-Mormon) denomination...
And, of course, this will only grow as more move further from their Mormon roots.