"System" disk space keeps filling up for no reason

Perhaps there is some sort of problem that is generating a lot of log messages? Also look for some processes that are using a lot of system resources - Activity Monitor.app might be a good start - click on the "CPU" column until you see a down arrow, signifying that the sort will be from largest to smallest (the up arrow shows smallest to largest, which we don't care very much about). Look at the /var/log or /private/var/log folder for really big log files - in your case after a couple of hours after booting up you might see something that is over 100GB is size if it's there. What apps do you have running on the system most of the time?

Good luck...

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

Then we can examine the report and see if we can determine if ABTRemoteAgent is on your machine and what's it's doing.

What could be filling up your hard drive (but should be deleted as space is required) are local snapshots for Time Machine.

You can see how many you have by entering the following command in Terminal:

tmutil listlocalsnapshots /

Note there is a space after the /.

This is what I have on my iMac running TM full time:

If that is why you're filling up your space you can remove them with the following Terminal command to remove a single snapshot:

tmutil deletelocalsnapshots 2019-09-23-170554

Note: it will take a while to complete the deletion, around 30 sec. or so.

If you want to delete them all at one time use this command;

sudo tmutil thinLocalSnapshots / 10000000000 4

I would not do this unless you can't find any other reason for the space being taken up.

EN1927:

Mark your last post as solved so others with the same problem will know there's a fix available. Glad you got it solved.

Thanks for marking my post as solved but it should have been your post as you solved the problem.

No wonder you needed to add an additional text section. I've never seen so many crash reports before. Not even combining all of the Etrecheck reports I've seen here. There is definitely something amiss here.

What do you use CCCleaner for? Have you ever tried to remove an app that came with the system? If so you might have removed files that are needed by the system and may be why the crashes

It's those crash reports that might be filling up the hard drive.

I'd start with booting into the Recovery volume (boot with the Command + R keys held down), selecting Disk Utility and running Disk First Aid on your drive.

Next download and apply the macOS Mojave 10.14.6 Combo Update followed by macOS Mojave 10.14.6 Supplemental Update.