Throwing shoes, or showing someone the soles of one's shoes, is considered highly offensive in the Arab world.
The bus was playing Jewish music and may have had Chanukah decorations, possibly children wearing kippot, making it obvious that it was filled with Jews.
The incident was reported to Metropolitan police.
I have not found any news stories about this yet.
(h/t Jonathan)
UPDATE: The Daily Mail has the story, and adds that the bus was full of Jewish teenagers and that the police are looking into this as a "potential racially-aggravated public order offence." (h/t YMedad)
UPDATE 2: TheJC has all the details.It was a Chabad bus with young Israeli and British Jews. They stopped to dance on the street and the Arab men started intimidating them.