I think too that one must know that they are abandoning the Truth for a heretical sect. I imagine in many cases their ignorance of Catholic teaching keeps them from being fully culpable and therefore they are not apostate by the strictest definition.
Just my opinion which I gladly submit to Mother Church in such matters.
I would think there must also be an element of "willful" denial of the Truth. I have known Catholics who "converted" to some Protestantism who seemingly did not have any great understanding of Catholic teaching. This of course plays into the Protestant claim that they "teach the Bible" and stress things like personal Bible study, etc.
Such individuals may also have been disenchanted with some aspect of the Catholic church of their upbringing, and it must be admitted that any church can have deficiencies. Thus it may well be that such individuals are not guilty of a "willful rejection" of the Church but rather may be reacting to specific problems that caused them to look elsewhere.
I am not excusing their response to the situation, but would rather tend to consider it a matter of ignorance on their part and perhaps some practical failure on the part of the church of their upbringing.