In case
a330pilotcanada's post wasn't sufficiently clear...or, in case I've missed the boat altogether, we've now seen the second such incident in as many weeks.
AvHerald's report of the latest - with reference to the first:
Incident: Air Canada E170 at New York on Dec 9th 2012, descended below safe altitude
Perhaps the cause of the second is written on the bottom line of the first:
The Canadian TSB reported the airline conducts an investigation into the occurrence.