From what I can make out - and it is quite complex - the following are some of the factors involved
- time of the flight under threat (if they are cancelling 20% of service these need to be spread along the timeline involved and this may reduce options a lot)
- ideally not a night stop or crewed for night stop since that ends up disrupting services for multiple days
- bias against flights with a lot of "on carriage" passengers (connecting passengers)
- can alternative service meet the demand by upgauging a planned A319 to a A321 or even longhaul jet?
- domestic services are prone due to alternative air and train services as well as avoidng a long aircraft outage, however they often have on carriage
- also bias towards services which aren't running full in both directions
- avoidance of one a day and longer service routes such as ATH and LCA.
- rare for longhaul to have proactive cancellations, excepting Doha and sometimes JFK. They may take out SJC or BWI in favour of SFO/IAD.

I remember once they proactively cancelled a LCA service, unaware that it was actually the only service for 48 hours and night stop crew, so they then quickly restored that service and rang up customers knowing that many of them would be confused.