I personally think it's because if the show DOES get cancelled, they have more than one way of saying "The mother was always <insert name here>". I mean it does seem to have come from nowhere, and all this about Sarah Chalke's character being able to only go to one party in the last year on top of the prolonged bump into a random woman Ted encountered on St. Patrick's Day, just all seems like 'fail-safes'.

I don't think people should be reading so hardcore into them.