Try going into that specific community, and in the sidebar towards the bottom you’ll find a button for “modlog”. It should have all moderator actions in there, and they usually give a reason for why they did it. Though they don’t actually have to or anything, it’s just up to them.

Also maybe want to check the modlog for the entire Instance, on the Instance sidebar.

I’m not sure if the Signal app has things in different places in their specific UI, incidentally. But there should be sidebars with modlogs somewhere.