Far-right extremists coopted a vaccine mandate protest by cross-border truckers, fueled by money funneled to them by sympathetic American separatists--among others, and aided by sympathizers within the Ottawa police force, they were able to establish a large blockade in the heart of the capital in sufficient numbers to eventually outnumber available law enforcement. They followed that with intimidation of any police who attempted to control their behavior or clear them from any area and brought their kids as human shields in a bid to deter police from taking action. Many police looked the other way in general, though, so not much to worry about there.
The truckers thought they were protesting vaccine mandates for cross-border trucking, but it was crafted into an over-arching demand to drop all COVID-related public health mandates and to replace the elected government with an ad hoc committee. There is evidence that leaders of the protest are connected with a group of 4 caught with weapons and body armor who are accused of a planned assassination of RCMP officers. One piece of body armor had patches belonging to a group called Diagalon, who are an anti-government separatist militia with broad reach in the US and Canada. If the protest ever really was about truckers and vaccine mandates, that stopped being true some time ago.
Something a lot of people don't seem to get about this (although they should,) is that in Canada, similar to the US, there is a legally defined separation between the rights/powers and responsibilities of the federal government, and those of the provinces. Depending upon the circumstances, there is a tendency among people of all political stripes to assume the interpretation of those responsibilities that is most favorable to them at that moment. In this case, the organizers of the protest took advantage of that to successfully push the misleading idea that it was up to Trudeau to resolve the issue when it was a fundamentally provincial/municipal problem, however much we all might prefer otherwise.
Without facing later criticism for over-stepping the federal/provincial boundary, he was very limited in what he could do without the powers granted by the Emergencies Act, but even now critics are still saying existing laws were sufficient to deal with the situation--they obviously weren't, as it can now be seen that far more of the protestors have cleared out and their numbers continue to thin. In that light, he didn't have sufficient reason before, but the addition of these Diagalon would-be assassins gave Trudeau all the justification he needed, IMO.