The defrost cycle is a normal part of how an air-source heat pump runs in cold weather. As the outdoor coil absorbs heat from chilly, damp air, its surface drops below freezing and frost forms on it. A frosted coil can't absorb heat well, so the system periodically melts it off: it briefly switches into cooling mode, sending warm refrigerant out to the coil to thaw it, then switches back to heating. During those few minutes you may see steam off the outdoor unit and feel the indoor air cool slightly — which is why backup heat sometimes engages to cover the gap.
Defrosting is unavoidable, but how the system decides to do it matters. Older units use a timed defrost — every so many minutes whether or not there's frost — which wastes energy melting a clean coil. The Department of Energy recommends choosing a heat pump with demand-defrost controls, which sense actual frost and defrost only when needed, reducing both heat-pump and supplementary-heat energy use.
For an install, the practical points are placement and drainage: the outdoor unit should sit where defrost meltwater can drain away and not refreeze into ice under the coil, and it should be raised on a stand or pad in regions that see snow. Frequent, very long defrost cycles in mild weather can also signal low refrigerant charge or a failing sensor — worth a service call rather than ignoring.