If you look at serious threats to the council. Farrel and Nenshi earn the bulk of the threats. If the issue was highly correlated with sexism it should be the female councillors receiving most of the threats.
https://www.google.ca/amp/www.cbc.ca/amp/1.4259271
The data in this article is poor as it doesn't compare the same
Time line but in 2013 Farrel received lots but it has since dwindled whereas Nenshi has remained steady. There is no mention of DCH receiving any. I'd be interested to see the raw data though.
From the surface it appears it is a result of activity level and the causes chosen. Now the type of threat to Nenshi likely involves Muslim hate speech and the type towards Farrel being anti feminist hate speech but that doesn't mean that is what is driving it.