Apparently back in 2006, the Canadian Auto Workers union agreed to concessions to reduce costs, convert to a more flexible manufacturing process and guarantee that future vehicles would be built at the plant. They're now claiming that GM wants additional concessions to secure those future vehicles, and all GM has to say is that "future products for Oshawa are in flux." According to CAW, it has until this fall to make those future product not in flux, or else the General will have another strike on its hands to deal with.
[Source: Automotive News, sub. req'd]