It's something i've hearing a lot of people saying

Hmm. There have been a few claims flying around Reddit for a year or two, that come up in this sub fairly regularly. On the off-chance you're referring to some maps or videos, do check out this post from last week: How accurate is this image about the Crusades and Islamic conquests? and this older one I came across an infographic marking and comparing the battles of the crusades vs those of the muslim conquests, how accurate is this?, which also links to a post addressing the "the Crusades were a defensive action" claim. If neither of those are on point, if you can give more details about what you've been hearing, someone can address that specifically.

As for motivations of the Crusades, a great place to start is the FAQ section The Crusades | Cause and Motivation, particularly the first post, Who started the Crusades? (Each one) by /u/Ambarenya

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