I've got a friend that used to raid with us and was actually really good. When we transitioned from Ulduar to TOC he left the guild and joined another guild and ended up becoming the raid leader. They're not the top progression raid guild on the server but they're very respectable and are well into heroics at this point. In fact that 10-man heroic I ran last week was his group and I ended up running the weekly with him and a couple 5-mans on my Druid and afterward I got the "you should leave xxx guild and come raid with us". I laughed, but not because I thought it was funny. I laughed because I've thought about joining them many, many times over the last few months. I stayed in my current guild because of the core. I like the guild leader and the officers a great deal and they've always been strait by me and I like that feeling of not just being a body. I might have to accept that I will probably never get to do everything I want to do but some times you have to give up something to get other things you want more.
Now this isn't what I'd really consider being guild poaching, but it's a good enough example to speak about something that has been going on for as long as there have been MMOs and if anything, intensifies during the summer doldrums. Guild poaching is the act of actively recruiting another guilds members, and usually the better geared or better playing members. To make matters worse, it's usually done by top progression guilds who treat the smaller or less progressed guilds as a minor league whom they can farm talent from at will. Most of the time these guilds are trying to replace retiring players, but just occasionally the tactic is used offensively to halt or stymie progress in competing guilds.
No one can make anyone leave a guild and join another, but if the price is right they usually will given the chance. And personally I'm not in favor of that sort of thing. I have no beef with someone leaving a guild of their own accord, but I do with another guild actively recruiting players out of someone else's raid.