I'm no expert, but I thought Garvey and DuBoise had more substantive disagreements- in addition to personally disliking one another. Garvey was a nationalist, and DuBoise wasn't, for instance. This may or may not be personal, but I also got the sense DuBoise and the other "elite" Blacks didn't respect Garvey much as an intellectual.
All of this is more or less valid, although the intellectual thing isn't totally true, since Booker T. Washington and his sympathizers were as elite as it gets, and definitely crossed over strongly with Garvey's, though their goals and ideas weren't the same at all.
Garvey's nationalism wasn't just about Black unity, but an active "back to Africa" movement, which Du Bois didn't see as a viable or productive route, despite his own pan-Africanist views and visions of Black unity.
Garvey's willingness to work with the KKK was another major factor, both as a specific set of events and in terms of what it symbolized in both men's ideologies. Du Bois thought such an idea was absolutely insane, while Garvey thought it was more pragmatic than trying to build coalitions with open-minded white folks.
At the end of the day, though, what sparked their mutual dislike was mostly personal- they were both trying to attend the same Peace Conference in Paris in 1918, and some strange scheduling problems + missed connections that prevented Garvey from being able to attend led him to accuse Du Bois of deliberately sabotaging his own agenda in order to set himself apart, and then things escalated from there. Garvey rallied his followers against Du Bois and accused him of being dependent on whites, and Du Bois published a piece on the Black Star Line in Crisis that, while supportive of the idea, also exposed some of Garvey's own authoritarian tendencies and deliberate exaggerations about the success of his business. Then, after Garvey was arrested for mail fraud, Crisis ran an even more expository piece basically covering the complete collapse of the Black Star Line. After that point, their mutual animosity was pretty much cemented. They both called each other awful and petty things and in retrospect, much of the exchange between them seems pretty childish overall. Du Bois eventually called him insane, commented on his physical appearance, etc, while Garvey called Du Bois a "mulatto monstrosity" and claimed that he was tapdancing for whites. Basically, very little of what either one said about the other was true. Anyway, after Garvey was deported, Du Bois eventually wrote a piece in which he expressed some regret and wished Garvey the best for the future, but Garvey never really cooled down and continued to badmouth Du Bois/the NAACP until his death.
