It is likely those addons themselves have a minimap button setting to show/hide. That is to say, their settings need to match the one in SexyMap. If Spamsentry is trying to show its button, while SexyMap is trying to hide it, then the last addon to show/hide it wins.
It doesn't have to be a minimap-related addon to move a button. For example, a clock addon (that has its own display) could have an option to show/hide the default minimap clock, and its show/hiding interferes with SexyMap's one.