Inadvertent button presses, as mentioned, are strongly suggested … bin there, dun that … my "favourite" is to press the AE-Lock button as I lift the camera to my eye, with the camera pointed in the wrong direction, then making an exposure without noticing the asterisk in the viewfinder!
If you've got metering linked to the a/f point and you've managed to accidently shift the a/f point with the joystick. this can also cause issues.
My "workaround", especially with my K-3iii, is to set up a User Mode (or several) so the camera always switches on in exactly the same configuration.
That way it's the work of a second to switch the camera off and on again to reset everything, not to the default settings, but to what I want
Using the Operation Lock feature can also help
However, I would suggest careful examination of the EXIF of the troublesome images to confirm that there was indeed an unexpected variation in the exposure settings or unintentional positioning of the a/f point.
If everything appears "normal" this might indicate problems elsewhere … dirty contacts or sticky aperture would be my initial checks.
Good luck