Source - http://www.politifact.com/texas/statements/2017/jun/29/shawn-thierry/shawn-thierry-says-texas-leads-nation-mothers-dyin/
I'm gonna go out on a limb and blame it on Presidents George W. Bush and Obama ad-Dajjal. Why do I say this?
Take a look at this graph of US deportations by year:
Note the deportation rate more than doubled between 2002 and 2009, and then stayed at that high rate (~400,000 per year).
My guess is that immigrant mothers, worried about possible deportation, stopped going to clinics for prenatal care around that time frame. This led to untreated problems associated with pregnancy which caused the higher mortality rate.
Just a thought. Needs confirmation, of course.