Looks like they changed schedule, or cancelled on that day. Or it's way overbooked and no longer available for sale. However in the latter scenario they would not have done the involuntary change presumably.

At the very least instead of being lazy and only changing the first leg they should change the second leg to the same day.

However, if calling AC, they should be willing to accommodate on the same day if insisting. Quoting the tariffs if needed (having checked them first obviously).

I had a similar situation but with a Europe Latitude pass. The kept my YYC-YYZ but switched the leg to Europe to the following day. Insisted that they would only rebook me as per pass rules, i.e. entirely on AC. Which arguably is a violation of the tariffs. I probably should make a case that pass rules apply up to issuance of a ticket but that once the ticket is in place, tariffs apply, regardless on how the ticket was obtained.