According to tweet author[1], this happened only with Youtube, and was not related to captive portal mechanism whatsoever. So I would side with her on the why: Poor plane internet access was overloaded by videos streamed from Youtube and somebody hacked together a very ugly solution that's going to have bad consequences...

[1] https://twitter.com/__apf__/status/551132865555996673

https://twitter.com/__apf__/status/551096550516994048