I wish I could be more help but, barring malicious software and/or software
corruption, the cursor in Word (and Excel) responds to input from some
external source - keyboard, mouse, touchpad, microphone, I don't know what
else you might have - bar code reader perhaps. If none of these are
receiving human input perhaps there is a physical fault with one of them.
As no-one else has responded here, I presume other (known) causes are few.