Actually, WTC 4, 5, and 6 were all damaged quite severely. #5 mostly on the south facing side away from #7. #6 had a hole in the middle of the building and severe damage extending to the north towards #7. #7 was completely collapsed. It's completely plausible that parts of WTC #1 fell on #6 and into #7 causing the collapse. I'm not a structural engineer nor am I an expert in falling buildingolgy; but many of the buildings around the site were structurally unstable and on the verge of collapse (1Liberty Plaza, for example). On the surface, they appeared to receive minimal damage; yet it took several weeks to confirm they wouldn't fall as well.
Perhaps #7 was built poorly. Perhaps debris fell more directly on it than it would appear. Perhaps NYC OEM blew it up because they really wanted a new office. Perhaps there's no real mystery at all. But, I guess it's fun to postulate.