Unexpected delay on pin sleep wake up

I am using an XBee Pro 900 in XBP09-DP mode

I set my XBee to pin sleep mode (SM = 1) .
When I set pin 9 of the XBee high (3.3V), it immediately goes into sleep (as expected).
When I set pin 9 to ground, it seems like it takes a random amount of time to get out of sleep mode, ranging between 1-8 seconds.

Where is this wake up delay coming from? I want to wake my XBee immediately (within a few milliseconds is ok) when the pin 9 detects a change.

EDIT: I changed USB adapter boards and it works fine. I was using the Parallax XBee USB adapter before, which was giving me a random wake delay for some reason. Once I changed to sparkfun explorer boards, my Xbee wakes immediately when pin 9 is de-asserted

Glad to hear that you got it figured out.