I don't think its random - it rather depends on which is your final destination. I remember that once last year the ZRH BA check-in agent (they are employed by the handling company Dnata/JetAviation, but work constantly for BA) didn't put a priority tag on my bag when travelling in CE/CW to YYZ via LHR. When I asked her to do so, she answered it didn't matter for YYZ, as there was no "separate container on the YYZ route".

I don't know whether and how she knows, but at least it's fairly consistent with the OP's observation.