I don't know why Singapore ATC is always so pedantic in asking the exact reason for the return to WSSS. It's not the first time I've heard this level of interrogation by Singapore ATC for aircraft diverting to WSSS. Is it for security reasons? They don't appear to be welcoming to diverting aircraft.
As for Tiger's callsign, they changed to 'Go Cat' a few years ago because their previous callsign sounded too much like another carrier's. And now since Air Asia changed to 'Red Cap', they're back to the same problem.