iOS 11.4 and the upcoming macOS 10.13.5 High Sierra have a feature called Messages In iCloud, it syncs all your Messages between all your devices...
One of the things that was supposed to happen in macOS High Sierra was syncing of all texts in the Messages App on all devices..
I know people that have inadvertently turned on iCloud syncing of Desktop and Documents Folders when they installed Sierra. Then, to their surprise, they run..