As far as I know antivirus definitions of AV programs can be very easily conceived by other AV programs as VIRUSES (well... the signatures and suspect strings are there...), and you guess what will happen if several thousands of "viruses" will be detected in one go!
Of course you can set up the AV programs to have the other's definitions in their exclude list, but I guess that you will still encounter problems, say with the iAVS updates.
Some AV's like Norton leave the definition files in some folder even after uninstalling them (typically under program files/common files/something), and that thing can cause similar problems when you install another AV.