A lot of potential reasons:
a) Impatience: those in for an Oct 31 div date don't want to wait till December
b) Buy on rumour, sell on fact.
c) This stock has outperformed the market so maybe re-allocating funds
d) Those without the benefit of franking credits
I'm sure you can think of some more.
For those prepared and able to wait another month though it would seem lucrative to stay in!
