Draenei shamans came about when the shaman trainer found that the light no longer answered his prayers, he went out for a walk, sat down near a lake and some trees, while smoking a spliff he noticed that the elements were talking to him (duuuuude) and then became a shaman.
I suppose there's no real reason any race can't become any class... All races have magical as well as physical stength to some degree. If a draenei orphan was raised by a group of warlocks, he could very probably become a warlock.
Splinter cell groups of races often distance themselves from the main group. Night elf mages aren't (lore wise) affiliated with the darnassus crowd. They are kel dorei ( i think) deciding to join up with them. Dwarf shamans come from the wildhammer faction of dorfs. They decided that on their own they couldn't handle deathwings beef and teamed up with the ironforge aka bronzebeard dorfs we know in game.
TLDR: Rebel race members could and can set up their own group and become w/e the fuc they want.