The majority who come out of the JWs, have been given this title because of leaving them.
But is the "word" as bad as it seems, the Wikipedia definition is as follows
Apostasy - is the formal religious disaffiliation or abandonment or renunciation of one's religion
The word apostasy can sound very harsh, cruel, vindictive and definitely judgmental, causing some families great hardship because of this.
My wife who was a catholic before joining the JWs must according to this definition have been apostate because of her abandoning the Catholics. Yet still she was accepted with open arms into the JWs
Therefore why, with some mainstream religions can you NOT be accused, yet the JWs can use this phrase directed at the person (this made publicly), as if you had abandoned GOD, rather than abandoned their assumptions.
KT