Maybe you weren't as good of a person to be around as you thought you were?
Maybe they have unfulfilled wishes that another FC promised them? (raid clears, etc)
Maybe they made a nice friend in another FC and decided to go there?
Maybe they're a flaky jerk?

Who knows. Different people have different reasons.

However, if you're having trouble keeping people, multiple people, then you may wish to set your pride aside and look at yourself and your FC, because those are the common denominators in all of these cases.

Judging from how you worded your OP, well.... it makes me wonder if there might actually be something else going on that you're oblivious to.