Microsoft has begun forcing computers running the Windows 10 April update 1803 to upgrade to the latest version known as the May 2019 update or 1903. Back in May they announced that the mandatory update would start early July. Microsoft mainly cites security as the primary reason for the mandatory upgrades.
Found on Microsoft's Update Dashboard "We are initiating the Windows 10 May 2019 Update for customers with devices that are at or nearing end of service and have not yet updated their device,""Keeping these devices both supported and receiving monthly updates is critical to device security and ecosystem health.". Currently there is no way to know when a given PC is scheduled for the mandatory upgrade.
Users who have already encountered the upgrade will be given a notification and a few options to either restart now, later, or postpone up to 35 days at which time the update will automatically install. Depending on the hardware specs of your computer the upgrade can take upwards of a few hours.
The update has been blocked from specific computers that have hardware incompatibilities with the 1903 update most notably Microsoft's own top of the line Surface Pro 2.