Because of the increasing numbers and complexity of new PDFs, from version 5.7.0 onwards the memory requirement declared by LHAPDF has exceeded 1GB! Apparently Mac OS X does not like this. A solution is to build LHAPDF in the "low memory" mode by specifying the "--enable-low-memory" option at configuration.
i.e.
configure --prefix=install/directory --enable-low-memory
make
make install
Beware however that this is fine if you are only using single PDF sets but may cause a degredation in speed if looping over errors sets.
A new version of LHAPDF is being worked on to deal with these memory issues.