The removal of the Fiesta was publicized in the latest issue of the SCCA's Fastrack News bulletin, which stated that, due to roll-over risk, Fiestas "do not meet the requirements and are to be removed" from the HS category of eligible vehicles. H Stock (or HS for short) is the lowest category of vehicles certified by the SCCA for use in sanctioned events.
The disqualification applies to Fiestas from the 2011 model year onwards, and does not apply to the Fiesta ST, which remains eligible. Of course the Fiesta isn't the only model deemed ineligible for SCCA autocross events. According to Jalopnik, other small hatchbacks and crossovers including the Dodge Caliber, Fiat 500, Scion xB and iQ, Nissan Juke and Mini Countryman are also prohibited from competition.
We've reached out to both Ford and the SCCA for clarification on the issue, and will issue an update if and when we receive any substantial information.