I learned from @DaveR the reason that dimensions become disconnected and highlighted in red in LayOut:
Understandably, entirely deleting and recreating an entity in SketchUp that a LayOut dimension was connected to will result in a disconnected dimension(s) in LayOut because the new entity has a new Persistent ID (PID) that LayOut doesn’t know about yet.
Less obviously, there are apparently other ways that SketchUp entities can get new PIDs, and thus, disconnected dimensions in LayOut, without having been directly deleted and replaced by the user.
I have read in other posts that some plugins will generate new entity PIDs when they are used to modify existing SketchUp entities … and sometimes just updating and re-saving the SketchUp model will result in those plugins giving some entities new PIDs.
None of which make it any less frustrating when a bunch of red highlighted dimensions appear in LayOut for reasons that are not obvious or intuitive.