I noticed this too when a city was being besieged. Another thing I noticed was when I attempted to help out, I only had the option to join the defenders by sacrificing what was approximately half my troops (~40 out of 80), a lot of which were noble troops I had been saving up. These were randomly killed. The losses from that were much worse than from the siege battle itself, and will take a long time to recuperate. I wonder if this is part of the reason the nobles choose not to help, but I can't say.
Ideally, it would have been a lot better and thematic if I could have helped to relieve the siege from the outside of the city, but as part of the city siege, in a massive battle with two fronts. This makes sense, and would be much more epic; like the battle of Helm's Deep from Lord of the Rings, when Gandalf and the riders come along.