Why people leave....

Recently a friend of mine left our church and became the pastor of Open Door Baptist Church about 20 minutes away. He left on good grounds and I really think that God took him there. However, in our church he has about 15 relatives. Recently we've all been noticing that they're not at church on Sunday nights or Wednesday nights. We found out it's because they're all going over to my friend's new church. They said at the very beginning they were not going to go over there. The people at the new church requested that no one follow my friend over there and give him a year to get settled in. However, they are going and are now talking about joining. [The truth is, my friend doesn't want them to join because he thinks they won't recognize him as "pastor". Which, the family doesn't even recognize my pastor (their brother/uncle/etc.) as pastor.] I know it's hard when someone you love goes to another church or state or country. But is it the right reason to change churches? I was always told that the right reason to leave is if wrong doctrine is taught. They've been talking for several years on leaving, but haven't. I personally think they are using this as a way to get out of the church, but that's just me. (By the way... one person who's following him and missing 2 out of 3 weekly services is a deacon in our church and another is our Sunday school superintendant and 4 of them are choir members.) If they leave, they leave. Our church will not suffer because of their absence. God will give us new people, I know. So.... question... why do people leave? Is it right to leave to "help" someone in their new ministry?