I can imagine two, not necessarily alternative, reasons why Sen McGauran might have decided that now is the time to switch:

A. Being the Senate Whip for the Nats and trying to manage
Barnaby has made him realise there are only two logical alternatives
for the Nats:

(1) to sit on the crossbenches as Barnaby has threatened,
and try to actually achieve something for their voters, or

(2) admit
that they’re nothing but the “little red caboose at the end of the Libs
train” (thanks, Peter Andren, for that one!) and move forward into one
of the carriages;

and/or (probably “and”):

B. He’s read Antony Green’s analysis of
the future of the Nats in the Victorian Legislative Council
and that’s really reinforced the news that the Nats are heading down
the tube, expecting only one MLC if the 2002 voting figures are
repeated and maybe two in a “normal” election (Antony’s phrase). (This
was written about the draft regional boundaries, but the final
boundaries are the same – see here.)

It must be demoralising enough to watch your numbers slowly decline in
Canberra, but I can easily imagine that to realise that even worse is
about to happen in Spring St, where the Country Party used to make or
break governments, could be the final straw.