A classic performance cliff.
Some things degrade slowly as usage increases, while others perform fine until they reach a certain load, and then they just hit a wall.
Super over-simplification of what is going on:
There are a bunch of Activation servers. Those servers can support n activations per minute.
If you have n phones per minute (or less) trying to activate, everything is buttery smooth.
If you have n + some small number, there is a minor delay.
If you have 2n (or whatever), then not only do you have delay because it can't handle all the initial requests, all those waiting people also keep resending their activation request, which causes extra strain on the server/network/whatever, which causes further delays, which exacerbates the problem. It's a snowball/avalanche of traffic/requests.
Because we have a limited number of activations, it will eventually clear up, but the process will take much longer than it should, because some hardware somewhere is being overwhelmed by the demand, which is causing even more demand, as people keep retrying.
It's the same thing that can happen to websites, and was, long ago, known as "the slashdot effect".
