...or sticking keys. Check any shortcut keys that may control volume.
Another possibility....
Windows 10 Disable the Automatic Volume Adjustment Feature
Step 1. Press Win + R keys to open the Run dialog box, and then type mmsys.cpl and hit Enter.
Step 2. Navigate to the Communications tab inside the Sound control panel window. Then select the Do nothing option under When Windows detects communication activity.
Step 3. Click on Apply to execute this change. After that, restart your computer and see if Windows 10 automatically lowers volume.
or....
Uninstall the Realtek Audio Driver
Many users reported that the Realtek drivers are responsible for Windows 10 volume slider keeps going down. To fix it, you can try uninstalling the Realtek driver.
Step 1. Press Win + R keys to open the Run dialog box, and then type devmgmt.msc in the box and hit Enter to open this program.
Step 2. In the Device Manager window, expand the Sound, video and game controllers category, and then right-click on Realtek audio driver and select Uninstall device.
uninstall Realtek audio device driver
Step 3. Click on OK to confirm this operation and wait for the uninstallation to complete.
Once uninstalled, restart your computer and check if Windows 10 lowers volume automatically.
Good Luck!!