this isnt uncommon at all. people switch for a number of reasons but one big fat one is the job market is *&^%. law schools understand this (and many number among their faculty some profs who got a PhD first and then a JD and ended up teaching). gear your PS accordingly-- make sure you come off as someone interested in the law for various reasons and committed to making a go at it, not just a hyperspecified intellectual.
a friend of mine went right into nyu law from his phd-- hes like a tax history expert, has written a couple books, plans to teach.if your field is realted to LS, show how; if its pretty unrelated then build a case for yourself as someone making a career change later in life, just like any applicant who has been doing something un-law-y since undergrad.