I can only go by what the terrorists themselves say, which is that they do not want US forces on their so-called holy land. Do you have any reason to believe that they are lying about this?
That is the excuse from the Blame America First crowd, it is not the real reason, however I do understand how some come to that view by watching snippets of statements on the lamestream news without delving into all the aspects of the declared "jihad" aka "holy war". The official spokesman for the Al-Qaeda organization, Suliman Abu Rith has stated that this is a "struggle between Good and Evil" because "U.S. policy is biased towards Jews and Christians across the world".
While Bin Laden does mention occupation, he stated in a 1998 interview that "Surely, their presence is not out of concern over their interests in the region. ... Their presence has no meaning save one and that is to offer support to the Jews in Palestine who are in need of their Christian brothers to achieve full control over the Arab Peninsula which they intend to make an important part of the so called Greater Israel."
So it is not our occupation they are fighting against, but rather our support of Israel as God's chosen nation based on our beliefs in the Bible. They feel they must drive Israel into the sea, not merely because of the land at stake...but because their hatred is rooted in their concept that Judaism and/or Christianity is evil and they wish it purged from their midst, because they feel their religion is the one and only true religion.