When a cancellation is made, the available days go into the inventory. If there is a match to a waitlist, the computer makes the match and removes the days from the inventory. This is an automatic process. For lack of a better term, there is some type of "waitlist match file" that hold the match until a CM can get to it.
The computer cannot automatically make the new reservation. The waitlists are too complicated. Some require points to be borrowed, some require other waitlists to be canceled, sometimes the waitlists notes aren't very clear, etc. A CM has to finish the process.
It is my understanding that the CMs work the matches based on arrival date. So if there are lots of matches, yours may be in the file waiting to be finished. For example, the CMs will work the matches for next month's arrivals before those for next December.
Sometimes, when people call to check on the waitlist, their CM will look at the waitlist file and see their match waiting for processing. Some (not all) CMs will then go ahead and finish the processing. That's why it seems like you get your waitlist by calling. You don't really get it sooner, you just find out about it sooner.
Also ( and this is just my opinion based on previous experience running a large call center), working the waitlist is not as "important" as answering the phones. If the phones are really busy, "it's all hands on deck". So I think most background work is set aside when that happens. (That's what our center did). Perhaps only the most pressing waitlists are worked (arrivals very near) when the phones are busy. In calls centers, one of the most important measurements is hold time in the queue. Long is bad!
The other thing to know about waitlists is that
DVC will not hold days waiting for other days to become available. So if you are on the waitlist for 12/2 -12/8 and someone cancels exactly what you want for 12/2/- 12/6, those days will not be assigned to you in hopes that the last one will come through. If someone who got on the waitlist after you wants 12/2- 12/6s, they they will get them even thought you got in line first.