Why does my cursor move on its own?

A lot of the time, the “nipple mouse” on the keyboard is bad when that happens. You usually need to replace the keyboard on the Dell laptops. Some are modular enough you can pull a good one from a keyboard with other issues like bad keys, but most of them are part of the frame and it’s a keyboard swap.

The “easy” fix that costs $0 to resolve it, is to find the connection on the keyboard (on the E5*/6 series, check the onboard controller; on the E7-present Ultrabooks, keyboard connects to the trackpad PCB) and disconnect it.
WARNING: SOME MODELS LIKE THE E6400 ARE A ONE WAY DELETE AS IT MUST BE CUT TO DISABLE IT. VERIFY WITH 110% ACCURACY THERE IS NO OTHER FIX ON THOSE! The other caveat to this fix is it doesn’t repair the bad nipple mouse - it only stops the jumping.

If the issue clears up by doing that, then you know it’s just a keyboard problem. If you do find that it clears up the issue, you’ll need to decide the next steps you want to take, based on the model, and age of the system as well as the current residual value** vs. a new keyboard and any other potential repairs needed. For some, it makes sense to make the machine whole (Ex: Lat 7490), but for others the whole units cost nearly as much as the keyboard (Ex: Lat E6X30/20, E6X40, E7X40). Cost is usually only an issue on the independent controller type of keyboards before Dell integrated the controller into the trackpad - though exceptions apply like the E7440.

The cost problem hurts in many cases - on some systems like the E6440, the keyboard is easily around $40-50 for a genuine keyboard from companies like PartsPeople. The issue with these is if you shop for a no HD system properly, you can buy the whole freaking computer for about as much and move your HD over!!! Because of the cost issue, a lot of the older Latitudes get totaled out by bad keyboards due to high part cost vs. buying the entire system w/o a HD and moving your drive over, or the owners bypass it this way. UNLESS YOU CAN GET A GENUINE NOS KEYBOARD ON EBAY CHEAP, THEY DON’T DIVERT MUCH FROM PARTSPEOPLE IN TERMS OF COST.
SALE CAUTION: If you keep the old keyboard with the nipple mouse physically disconnected, disclose the problem and factor it into the price.

*If it was offered. Some had it as an option, while most never offered it.
**Covid note: At the time of this answer, even laptops like my E7440 (i7-4600U) that are so old nobody else but hobbyists who get a deal want them are worth more then they should be due to Zoom school. So yes, factor that post Covid hit onto the residual value average.