It is possible that as he pulled on the cable, he may have split the center conductor, or created a problem with one of the connectors (like raoul5788 said, especially if these connectors are crimps, as opposed to compression), or stretched the coax just enough so that one or more of the stingers no longer make proper contact.
Something to try, if you remove the coupler, and bring the STB to the end (that is too short) and try it, does it work? if so, then the problem is in the coax OR the coupler you added on to make it long enough, if the problem is still there, then it could be what I said in the first paragraph.