Why do a second PhD in a "neighboring" discipline?
I've seen two reasons for this.  The more worrisome reason is the one mentioned by EnergyNumbers in the comments, namely that some people just collect degrees.  In particular, they may feel having two Ph.D.s would look exceptionally impressive, or they may be unable to settle on a specialization, or they may feel that getting another degree is somehow preferable to getting a job.  These could be legitimate as personal desires, but they are counterproductive for a research career.

The other reason is to recover from a bad career start.  Sometimes someone's grades keep them from getting into a good graduate program, or they face other constraints (geographical or financial) in their choice of programs, or they fail to connect well with their advisor.  They may eventually get a Ph.D., but not under good circumstances, and a second Ph.D. might lead to a much better career.  The difference between this case and the first case is that these students don't actually want a second Ph.D. as such.  They regretfully feel they need one to achieve their career goals, but they wish it had been their first and only Ph.D.

Recovering from a bad start is a less worrisome reason to seek a second Ph.D., but of course the challenge is figuring out which students have moved beyond their past difficulties and which have not.