Actually, it's more like the then mayor of Montreal (Coderre, ex-Federal Liberal minister) unilaterally and without consulting anyone came out against it after viewing a few crackpots crashing the National Energy Board hearings. Around Montreal, the views were split, at about 50/50 for or against the pipeline, but overall, in the whole province, the surveys I have seen were 60% in favour of the pipeline to 40 % against. So outside of Montreal,there was good support.
The provincial Liberal party, currently forming the government, quickly sided with the mayor of Montreal only because their strong base is located in the Montreal area.
Interestingly enough, the best argument in favour of the pipeline that got some traction outside of Montreal was that such pipeline would immensely reduce the number of rail tanker-cars transiting through their area (they remembered Lac Megantic). The mayor of Montreal - or most Montrealers, seem unaware that in excess of 100,000 such tanker cars of crude/oil transit through Montreal every year as it is, and this traffic is actually increasing. Most of it would be replaced by the pipeline. I know that the more quoted surveys allegedly show only about 35% overall support in Quebec, but those surveys - though largely quoted - were not found to be scientifically proper.
Moreover, the mayor of Montreal excuse for taking his position was, put mildly, bull. It was the danger to Montreal's water supply. The pipeline does not cross the St-Lauwrence anywhere upstream of Montreal's water supply's basin (Lake St-Louis, a lake on and part of the the River) It first crosses the Ottawa river a little west of the ON-QC border (this, if anything happens, would hold the actual oil West of the Carillon dam, with any overspill then being discharged into the River well east of Montreal, after going through the Deux-Montagnes lake - Milles-Iles and Des Prairies rivers.) Then, it recrosses the Saint-Lawrence near Quebec City.