I might be thinking of the wrong bridge, but I assumed this was because there was a sharp corner on one side of the bridge and it would be impossible to see somebody coming round the corner (hard enough when I was out running). But the reason shouldn’t really matter, when asked to dismount, cyclists should dismount, even if they can’t immediately see the logic.
But I wouldn’t want to be seen as anti-cyclist - 95% of cyclists behave admirably, at least elsewhere on the roads and pavements, even if not on this bridge.
No idea I’m afraid. But here are the websites for both FHSoc and SydSoc which will contain contact details for the principals as well as detsils on how to join and play an active role. I don’t know about FHSoc but I do know that SydSoc has over a thousand members but only a handful of volunteers who take part in committees and campaigns. Speaking from past experience, it can be a relentless and thankless task. But I don’t want to put you off volunteering to get involved, Stuart!