The explanation as to why Montreal is a fairly busy tech hub likely has to do with salaries here vs. the expectation for equivalent talent in the US. Most companies here are actually American companies, and the only reason their offices are located here is because even senior developers tend to earn < $85k/year CAD, with $60-70k being more common. That's $48k to $70k/year USD vs. the $100-200k+ USD they'd have to pay within the US. Government incentives (looking at you, SR&ED) reimburse these companies a significant portion of developers' salaries, making it even cheaper. Canada is, for many American businesses, essentially a step above outsourcing to India.
There are a lot of game studios and other tech corporations like Google, Shopify, and so on.
Also you can find many talented programmers in Montreal. It is particularly enjoyable if you are looking for a really diverse city both culturally and technically.
More recently, a lot of investment starting to go into AI: http://montrealgazette.com/business/how-montreal-aims-to-bec...