Hiring a PhD student who left another PhD position It's the first time I'm recruiting for a PhD student. I advertised the position and have received a dozen applications and there are still 2 weeks left on the 4 week announcement.
What caught my eye is that I have two applicants who are already PhD students at other (lower ranking) universities. They both have very strong CVs and even some good publications. However, they haven't included any references from their current university or the reason they want to leave from there. One is a second year PhD and the other a third year. Both have similar topics to the one I'm advertising for.


Is it OK if I ask them directly why they want to leave and why they haven't included any references from the place they have been for 2-3 years?

Should I try contacting their current supervisor and ask for info? I even know one of them personally. I don't know if I should (could land the student in trouble).

In these situations, don't you feel that this person could suddenly leave if he/she finds a better position? Can I really trust him/her to stick through a 4-year project?



P.S.: My wife switched to another supervisor 6 months into her PhD because she really couldn't get along with her 1st supervisor, so I understand these things happen. I want to give these candidates a fair chance, but I want to cover my bases as well.
I would contact the students directly for answers before contacting the supervisors. This way, you can get some feedback and further information before deciding if you want to contact the supervisors. 

However, I would in any case not take a student from the lab of an acquaintance without the "blessing" of the acquaintance. Otherwise you risk setting up an unnecessarily antagonistic relationship between you and the other advisor.