Certain areas are facts other areas are conjecture.
Pope Urban II gave an emotional call to arms after an appeal from the Byzantine Emperor to help repell invading Turks
The recapture was in 1099 - the lands (including Jerusalem) were lost in 634
I think it's hard to see the first crusde as a 'response' to anything in Jerusalem
You have to put yourself in the mindset of medieval Christians - it was a great woe that the Holiest city of the 'true' religion was in the hands of infidel muslims and I have little doubt that the Pope thought he would gain great merit with God in recapturing it.
I'm also sure all the Christian Kings thought they would absolve themselves of all their many sins by doing this too.
http://en.wikipedia.org/wiki/Pope_Urban_II#Crusades (Urban's call to arms)
It's hard for us to think what was in these people's minds - it's tempting to think of them as cunning manipulating the religion ofthe common people for their own ends.
But they really believed this stuff - in hell and damnation and Salvation and infidels and miracles and witches - they were very superstitious and you cant exclude that from their motivations.