In simple terms. Endless blocking.
To be more precise. Here's a senario.
Bitdefender blocks W32/Virut. While processing its deletion, Norton detects W32/Virut running (Well it is running because BitDefender is currently deleting it.
Norton blocks W32/Virut deletion by BitDefender (Because Norton program thinks BitDefender is running the W32/Virut.
W32/Virut deletion process gets transfered to Norton.
BitDefender will block Norton because it's running W32/Virut (Norton is applying deletions methods on W32/Virut that's why W32/Virut is running).
Repeat process.
Your Anti-Viruses will eat your memory like crazy.