I would hazard a guess that the "field ex out west" is the newly commenced Brigade Training Event (BTE) which will take place at the new CMTC in WATC. One Bde per year will be run through it, with 2 CMBG being the lucky first. They are due sometime in the Apr/May timeframe, if my memory serves.
What you are witnessing is why it is so difficult to involve reservists in a Roto "0" yet also so easy to include them in Roto "1x". The TO&E for a Roto 0 will continue to evolve up until the day the last Chalk departs Canada...well...actually long past that as well but until the TO&E is at the 80% solution, nobody is going to issue any tasks for augmentation simply because it might end up the augmentation is not required. After the first couple of Rotos things settle in and the TO&E pretty much gets cut and pasted from the previous Roto. Zoomie was right, what happened was a staff check very early in the game and should not give rise to any false hopes.
It‘s a trade-off but the Army understands the sacrifices the Militia pers make to go on tour and the consequences of dropping said Militia pers from the tour at the last minute. This is why when Op Palladium Rotos are mounted the directive, at least in LFWA, is that if a Militia member passes all of the training and has the appropriate quals, they will have manning precedence over a Reg Force member. This happened in a big, big way for Roto 11. Reg Force member is dropped from the tour, s/he carries on with what they would normally do and is available for other tasks which arise. Militia pers is dropped, they have blown a year of school, don‘t have a job to go home to for 6 months, probably has spent a gazzilion dollars on gucchi kti and is quite possibly scared to attempt another tour in case they get dropped again with no compensation. Additionally, the Reg Force member dropped generally enters the replacement pool for the Op yet the Militia member is under no obligation to do so and won‘t be given a contract "just in case". Because of this a tasking brick will only go out to them when the TO&E is solid enough to make this possibility worth the risk.
So I suppose the conundrum faced is which you would prefer; ample notice with the very real possibility of being dropped from the tour (ask around about the number of pers who were on the bus, off the bus multiple times for Apollo) or shorter notice with a greater chance of going. I would hazard a guess and say it‘s 50/50 either way and would depend on how badly it was you wanted in on a Roto 0 vice a Roto 15.