Here is an article from the New York Times describing this in further detail. Here is a quote from that article, speculating as to the possible reason for blocking her re-entry:
Ms. Ghuman is certainly not alone in her frustration. Academic and civil
liberties groups point to other foreign scholars who have been denied entry
without explanation at an airport, or refused a visa when they applied. A
pending lawsuit by the American Civil Liberties Union contends that the Bush administration is using heightened security measures to keep foreign scholars out on ideological grounds in violation of the First Amendment rights of American scholars to hear them.