There are some theories, but nobody knows for sure. It just seems random. IRCC says there's a queue, but it's clearly not based on portal submission dates alone.
The leading theory is that it's based on eCOPR expiry dates, i.e. the sooner the expiry date, the sooner you get the eCOPR. This makes sense since I don't think they can issue an already expired eCOPR, so they might prioritize those whose eCOPR is expiring soon. The expiry date is usually the same as the medical expiry date.
However, I'm pretty sure someone with a July 2024 eCOPR expiry date already got their eCOPR even though there are people waiting (such as myself) with a June 2024 expiry date. Although, they did have to wait a very long time (50+ days) from portal submission as well.
So the queue is probably based on multiple criteria: 1) portal submission date, 2) eCOPR expiry date, 3) stream?
Or, the queue is a lie and they just pick applications at random.