When you’re working on a scene that holds transparent surfaces, it is quite possible that this either has no boundary, or you miss it, so you actually select what is behind it. It is not always noticeable. If you next drag, or delete the assets you expected, it will also take action on the unintendedly selected ones. Of course you could always revert by CTRL-Z - but only if you noted this directly. It would be helpful if assets - including landscape - could be protected agains being deleted - a warning would help
there is a free plugin called Actor Locker you should grab. This is an issue thats been asked for since the beginning of time but that plugin is only thing made to address it.