“Microsoft security bulletins are released on the second Tuesday of each month,” according to Microsoft’s website <https://technet.microsoft.com/en-us/security/bulletins.aspx> and your Windows PC is setup by default to check for and install these updates between their release on Tuesday and when you use your PC on Wednesday. It’s a best practice to let this happen for general use PCs. Some patches are larger than others and require you to wait while they install or may require a restart to complete.
This is unofficially know as Patch Tuesday. This Tuesday there were 10 updates. You can read more about the updates and Patch Tuesday at the links below.