DS-5535 is a form of extreme vetting. They will want like 15 years of travel history and some other garbage. The cases are sent to a central repository where a select team reviews them. It seems like Montreal hands out a lot, or were handing out a lot, but that may just be reflective of statistics because the community here is fairly active so there's talk about it. It does appear that Montreal is bad for skirting the buck and finding excuses for delays. It's 6 in one hand, half a dozen in the other and I do think, to their defense, it's a fairly busy consulate.
If your name is common, AP takes longer, in general.
If you've visited for a period of time or worked in a MENA country, AP takes longer and the risk of DS-5535 is higher.
If you have a job on the Technology Alert List (TAL), you are at higher risk of being handed a DS-5535.
If you're from a MENA country (sometimes even 1st gen Canadian) and have a TAL job, you're at higher risk.
Occasionally, it's just a random number generator and elderly or children are chosen. Crapshoot, really. It's just important to know about it so you can adjust your expectations, if needed. Many people are still just moseying on through with no issues. It's those that are unaware that are really blind sighted, so that's why people mention it.