Well, an anonymous person with access to some pretty slick testing equipment tipped off Rep. Jackie Speier (D-CA) about the problem.
Her office confirmed that somebody using a Thermo Electron Niton XRF testing gun found a lot of cadmium, sometimes used in yellow pigments, on the surface of the glasses. By law, no more than 75 parts per million of cadmium is supposed to be present in paint on kids toys. Speier's office said the amount found on the glasses was quite a bit higher than that. The source overnighted glasses to Speier's office last week, which then turned over the test results and specimens to the CPSC. She got several readings of more than 1,000 ppm cadmium on her XRF gun.
Seems like the answer to a previous question about at-home science — this blogger http://www.thesmartmama.com/toxic-cadmium-prompts-cpsc-recall-of-mcdonalds-shrek-forever-after-promotional-glasses/ seems to have been one of the anonomous sources.