heat pumps work by reversing the AC cycle to bring heat in, your condensing unit is cooling it's outdoor coils.
That's probably a pretty good guess on the defrost timer, the unit may need to be set to defrost more often
heatpumps tend to work worse as it gets colder outside and will be more likely to freeze up.
I rep the Bryant line, you can check
www.joebryant.net and download pdf data files for the condensing unit 661C would be a good guess, also
www.thermostatusa.com may give you some input as to programming your stat for more frequent defrosts if you've got a bryant carrier or totaline stat