It's because they're growing at an exponential rate - they don't have time to reconfigure the load balancers and end up with traffic bottlenecks. This *then* means that 1 server is providing the upload application to 2x, 3x, 4x, 5x (etc) more connections that it's supposed to. Sometimes you get lucky and end up on an 'un loaded' server and you'll have no problems. Other times, you're not so lucky.
So in English, sometimes they have problems keeping up with demand. More often than not recently
