This thread is all over the map with speculation. There is a press release from the Department of Justice that lays out what Assange is being extradited for, and it may help to read it first.

Apparently Assange helped Manning try to get into a classified computer system with a password that was not hers, and that is the basis of the extradition case: https://www.justice.gov/usao-edva/pr/wikileaks-founder-charg...

Related HN thread here: https://news.ycombinator.com/item?id=19634137