Freezing rain causes damage out of proportion to the amount of water involved. It requires a precise vertical arrangement of temperatures, and small changes to that arrangement produce entirely different precipitation.
The arrangement required
Precipitation typically begins as snow in the cold upper part of a cloud, regardless of what falls at the surface.
Freezing rain requires that snow to pass through a layer above freezing deep enough to melt it completely into liquid droplets.
Below that warm layer there must be a shallow cold layer at the surface, cold enough to chill the droplets below freezing but too thin for them to refreeze into ice pellets during the fall.
Why supercooled water stays liquid
Pure water does not freeze at its freezing point on its own. It requires a surface or particle to start the process, and clean droplets in the air often lack one.
Droplets can therefore remain liquid well below freezing while falling, in a state that is stable only until they touch something.
Contact with a surface provides the trigger, and the droplet freezes almost instantly on whatever it lands on, which is what produces a smooth clear glaze rather than a granular deposit.
How sleet differs by one dimension
If the cold layer near the surface is deeper, the melted droplets have time to refreeze in the air and arrive as small ice pellets.
Sleet bounces on impact and accumulates like coarse sand, and it does not bond to surfaces, which makes it far less destructive.
The entire difference between the two is the thickness of the near-surface cold layer, which is why one changes to the other within a few miles or a few hours.
Where the layered structure comes from
The pattern usually arises when warm air advances over a cold air mass that is entrenched at the surface, riding above it rather than displacing it.
Cold air is dense and resists being lifted, so it lingers in valleys and on the cold side of a stalled front while warmer air spreads overhead.
Terrain that traps drainage of cold air is therefore prone to prolonged freezing rain, while flat open country transitions to plain rain more quickly.
Why the accumulation does the damage
Ice adds weight uniformly to every exposed surface, and the load on a long span of wire or a wide tree crown becomes substantial at modest accumulations.
Wind acting on ice-loaded branches and conductors multiplies the stress, which is why outages in an ice storm continue well after the precipitation stops.
Restoring power takes far longer than after a wind event, because failures occur at many points across a network rather than at a few.