To save everybody a headache, I'll tell you why this happened before it blows up.
You see, Montreal's roads are generally quite shit for a number of reasons.
The underlying factor, being that we're cheap.
Since we're cheap, we don't make the roads very well (4x as thin american roads, 2x as thin as most roads in the rest of Canada)
Additionally because we're cheap, we don't buy the best of materials to create the roads.
This is the main issue why our roads are shit.
The reason being, is because we import a lot of limestone from Northern manitoba (despite the fact that we have it in major abundance here) since it is cheaper. This "cheap" limestone is used to process cement. However because it is generally impure, containing many metals and limestone aggregates, it is HIGHLY susceptible to chemical weathering. Side note: this is why our bridges are falling.
Lime and limestone aggregates are used for the creation of roads, don't get me wrong, however because the concentration of limestone within the slurry is low due to the many impurities, they just use more of it. This in turn raises it's concentration and frequency of impurities, making it a lot more susceptible to chemical weathering.
Now that you're aware of that, I'll explain something else.
Montreal's geologic history is quite unique. You see about 20 000 years ago, towards the end of the last ice age, Montreal sat under 2-3km of ice. Once the glacier had melted, due to the weight of the glacier, the earth around montreal was significantly lower than it was today and this meant that salt water from the atlantic came in and flooded everything creating the Champlain Sea. Salt water means sea life, and a lot of it. This sea life ends up dying, sinking to the bottom and forming limestone over the next 10 000 years. This is why the geology of Montreal and it's surrounding area are of limestone.
Fast forward to today and the underlying geology of today and we still have a lot of limestone underneath us.
Considering that it is spring, water is melting and all the salts and chemicals we use on our roads and are getting into the water table.
The water table is rising, however it is contaminated with chemicals such as the salts and other crap.
This is where the three things come together: the rising water table (that is contaminated) mixes with the underlying limestone, slowly weathering it away (since it is susceptible to chemical weathering) then the road above it starts to weaken because it too is being chemical weathered due to it's impurities and poor construction and the next thing you know, it collapses.
In nature this is normal and is actually how caves form but it also demonstrates as to why some caves can be dangerous.
When this naturally occurs, not from anthropogenic processes, it is called a karst landscape and they are quite beautiful.
TL;DR: Rising contaminated water table chemically weathers underlying rock and shitty roads to form a sinkhole because Quebec is too cheap to afford geologists or to create proper roads.