Was this cross dissolve originally placed across two clips, then one clip was deleted or moved thus leaving the cross dissolve and other clip? Look closely, does deleting the transition cut half of it's duration from the clip?
If so, when you delete the transition it looks like you are shortening the length of the clip, but it's because the transition has compensated for the missing the handle from the previously deleted/moved clip (which keeps the location and duration intact). After deleting the transition, the the clip is still the same length as it was before the transition was applied.
To get the duration back after deleting the transition: With snap to edges on, place the playhead at the start/end (whichever applies) of the transition, delete the cross dissolve, and drag to trim the clip back to the playhead.
Thanks a lot David - that was really helpful - turns out that in some of the instances the clips weren't being shortened from the original duration (ie before I added the transitions)... others I had to use the playhead like you said...