[heero_yuy]

I would think it's down to the ISP struggling to keep up with increasing demand.

Normally the CDN's are only handling relatively small amounts of data for each client and delays in feeding that data are relatively benign in effect. Youtube buffering for example.

Suddenly having to supply high density data streams on many connections in real time just maxes them out.

The source only has to provide a single or very limited number of streams which are then distributed within the ISPs network

Unlike cable/sat/freeview TV where there is a one-way data stream that is the same for all users. Streaming is an interractive process between CDN and client with each client having a bespoke stream.