1- Pitot probe covers or static port covers are not removed.
2 - Pitot or static hoses are disconnected.
3 - Hoses are leaking.
4 - Water trapped in the lines freezes during flight.
5 - Pitot probes or static ports are blocked by volcanic ash.
6 - The radome is damaged.
7 - Icing occurs on the pitot probes or static ports.
8 - Pitot probes or static ports are blocked by insects.
9 - Pitot probes or static ports are physically damaged.
10 - Air data pressure sensors fail.
As it pertains to an RV-10 static port.
1- I have never seen a static port cover for an RV (except by the painter)
2- If the static hose is disconnected you already have your alternate cabin pressure
3- Same if the hose is leaking, except now you get a mix of cabin pressure and outside static pressure.
4- Valid reason.
5- That's a bit of a stretch especially for the static ports.
6 - RV-10s don't have a radome and even if they did, the static ports are mounted on the fuselage sides.
7 - Highly improbable on the static ports, you would have accumulated ice everywhere else before the rear of the fuselage starts to ice over.
8 - Insect blockage occurs on the ground and on rare occasion perhaps on the pitot in flight but not on the static port.
9 - That would happen on the ground and you should definitely catch it on your preflight, people do knock off pitots but not static ports.
10 - If your air data sensor fails it would not start working again on an alternate static source.
# 5 is a valid reason to install an alternate static source and as others including me have mentioned, no harm or great expense to do so but absolutely unnecessary on an RV. A properly installed static line as per plans, follows the fuselage up about 10 inches from the ports and Tees off into a single line. For water to get into the system you would almost have to drown your plane. I suppose I will now get a lesson on condensation in the static line