Have experienced it and been accused of it. Never done it intentionally, though I can see why some people believe me to have done so. We had people come to us at one point and took them in, which upset their former CEO greatly, and I found that rather silly as the corporation they were part of was winding down. We also have seen people leave, and I think I can safely give the example of us going into stasis here with people moving into Veto. I could be all pissy over that and beat myself up and think Verone is a terrible person, but why would I? Things weren't good within PRETA at that time, and now they can be thanks to a fresh start, which is a win-win for both those who are back for the second incarnation as well as those who are now happier in a group that seems to satisfy their needs.
Poaching, intentional or unintentional, or even just opportunistically, is just a fact of life. You can worry long and hard about it, but it is wasted time. If people leave your corporation because they feel the grass is greener elsewhere, that is their fair decision to make. It may sting, and you may feel you have failed, or you're being mistreated by the party taking in your former members, but everyone pays for their own subscription and is thus responsibly, for good or for worse, for their own decisions. It is not your right to stop them. They are not your property.
Now personally I will always ask someone who decides to leave to be honest about why, because there may be an important lesson in it for me. Sometimes people and their priorities just change though. which is another fact of life. Do not take that personal. Realize instead you too change, which in the end, maybe also be a reason for people to leave. Never ever stop yourself from changing or make changes you do not like to prevent losing members. It is cheating yourself and it will backfire on you.
It sounds like a harsh thing to say, but if people want to leave for something that sounds better on paper but you can pretty much tell will be a gigantic failure, you sometimes just have to let them go and experience that. Burning our fingers on something hot is how we learn. And maybe they will not burn their fingers, and maybe they will end up having a great time and there will be no gigantic failure. Oh well. Would you have liked then to stand in the way of that? Could you really sleep knowing that members who stay with your corporation because they are afraid of your reaction are less happy than they would have been elsewhere?
Our egos want us to succeed and be the center of attention, but personally I believe a good leader is that person who does what he or she wants to do regardless of it having the back of 5, 50 or 500 members. Even if your corporation is the best idea ever and the savior of EVE and will make them remember this game as the best thing they ever did in life (god, I hope not, what a terrible life that would be).
Besides, there is this little thing called karma. If someone poached your members with something that sounds great on paper and the intention to run you into the ground, they also have to make it come true, meaning they are going to have to work for it very hard. From my experience poachers fall into the traps they dig for others sooner or later.
So, in the end, spend your time on positive things. Build a great corporation so people are happy. Focus on those that are staying and that are joining, even if you get just one person and they get 20. Numbers are overrated, if you play it smart, you can get very far. But you must want it. Nobody has ever had success land in their laps just like that. The must successful people are those who have also failed the most. But they always crawled back up, heeded the lesson that was to be learned, and kept going for it.