Poaching and jumping ship usually happens in the 10U-14U age groups. More-so at the 10U because coaches have a huge ego and families are ignorant of what they truely need for their DD to progress. At 16U-18U, it rarely happens because I think the parents, players, and coaches have finally figured out that honest, straight-forward communication is the key.
By this time, the families have figured out what they are needing for a team and the coach has a good handle on his/her capabilities and if that matches what a family is looking for in a coach and team.
Some of the younger age coaches do not realize that they do not have what it takes to get girls to the next level. They attempt to hang onto players who need to be on another team whose team skill level, schedule, and coaching will be better suited for a player.
But....this is not an excuse poaching. As for jumping ship, if a family was promised one thing and he/she is delivering something else, then I am out of there.
About 5 years ago, there was a team that would show-up at tournaments and get the living snot kicked out of them by every other team in the tournament. I talked with one of the coaches and ask why they came to these tournaments. He told me that they realized that their team was never going to be a very competitive team but they were there to give their girls a taste of competitive travel ball and also, hopefully, if a girl had the desire, they would see a team that they could try-out for the next season. Over the next couple of years, I saw girls that were on that team scattered out on different competitive travel teams. I think these coaches had it right.