The network wasn't ready when ntpdate ran. This can happen on more than
just wireless networks. I have personally seen it on wired RPI devices
that use dhcp and on virtual machines that have higher than usual
latency (i.e. full CPU and device emulation on a slower server). In
some cases, I ended up putting a sleep in /etc/rc.d/ntpdate to
compensate for this behavior. You can also do some clever stuff with
/etc/ifconfig.* in that you can run a command there, and that command
could be "sleep".
That works! I just added sleep 2 to the start function in ntpdate
and all's well.