Symmetry and SALC: choosing coordinate basis
In general, when dealing with characters tables and with group-operations, it is crucial to choose the right orientation of axes in order to get sensible results. I say in general because it can also occur that, for highly symmetric molecules the "wrong" choice (or better say unconventional) can give the right result. Almost the 90% of characters tables follow the "Mulliken" conventions stated in Mulliken's paper "Report on Notation for the Spectra of Polyatomic Molecules" (The Journal of Chemical Physics 23, 1997 (1955)). Whenever you orient your axes not in agreement with the characters table you are referring for performing operations, there is a high risk to fail and face strange results as you did (all orbitals mixing etc..). I hope my answer is general enough and helpful.