This is something that i brought up in the RWBY Volume 4 In-Depth Discussion and Spoilers: Down on Luck topic but would like to sum it up in a post.
Ravens semblance is predicting future danger, Salem rising threat. This is 2 things that have happened and has warned qrow about salem's weapons of fear.
She gives people a 1 time usage of a life save, she knows this is coming and acts on it when it happens. She wouldn't of just been strolling by and found yang about to die. However after the credits in vol 2, yang is shown talking to raven with her saying. " we need to talk". She could of been telling yang her future with Adam cutting her arm off and a close friend being hurt. A act that is not what it seems to be ( mercury's 'leg' injury) and other stuff.
Now the nukaleeve, this thing rolls by when a large amount of negative emotion has been shown. But raven being in a tribe has to lead her people, she rolls up to future villages, warns them of nukaleeve and in exchange for the exchange, supplies are given to the tribe. Should they refuse, nukaleeve attacks them.
Raven shows no emotion so that nukaleeve doesn't attack the tribe, that is why she left yang so young so she doesn't miss her when she is older.
Another thing is that the mask raven wears is a full facial mask that is in the shape of a nevermore. BUT when it was shown nukaleeve attacking Koroyuri, the only other grimm found there were nevermores. This is so that it doesnt further let the nukaleeve attack and wipe out the tribe. She warns the tribe of future danger, that is why they are always on the move to avoid this.
Hope this makes sense and would love to hear what you think of it