Windows Defender 99% CPU at Logon

Just logged on to a Windows Server 2016 Essentials machine for the first time in a couple days. Windows Defender immediately kicked off a Full Scan, basically bringing the server to its knees with 99% CPU usage.

Why does Defender need that much CPU for what should be a lazy stroll through the files? And why does it start when I log on to the server instead of running in the background, at night?

Defender CPU 1

After canceling the scan, I see that the last Full Scan was just over three hours ago, at 1:19pm.

Defender CPU 2

Why does Defender feel the need to run multiple Full Scans during the day, on the same day?

I wonder if there was a Patch Tuesday update yesterday that is causing these issues.

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