sounds like a short in your switch matrix.
when a certain switch is hit, it's activating every switch in a row or column.
obviously your slam tilt switch is in the problem row or column, so take a look at the switch matrix in the manual and look at the diodes in every switch in the affected row or column for a broken or missing diode. A ball could have hit the top of a switch and bent the diode so it's touching the middle pin. underside of the IJ mini playfield is a good example of this.
That's where I would start.
Put it in switch test and activate each switch one at a time and see if you get multiple switch closures on a single switch.

After you've done these steps, if you can't find a diode problem and you think it's opto related, just unplug your trough a other optos and see if the other affected switches are still shorting. Might help you narrow it down more.

Classic switch matrix problem, fortunately it shouldn't be hard to find in this game.

The only problem with doing a switch test like this is that you will likely never see a problem since the slam tilt will only be activated when a switch on the active column in row one is closed, along with another switch in that column that is on the same row as the switch in column 2 with the bad diode (which also must be closed). So it essentially takes three simultaneous closed switches to activate the 4th.

ccc matrix (resized).jpg

When I said this was going to be an easy find I had looked at row 2 of your switch matrix, and can tell you the problem is going to be one of 4 switches, 22, 26 ,27 ,or 28.

Sorry, this is incorrect as I over simplified this. For example activation of column 2 would always pull row4 low since this switch is always closed. If you had a shorted diode in switch 64 (drop #4 right), this low going signal would activate column 6. with column 6 active during column 2 time, closure of switch 61 (Drop #1 Left) would cause row 1 to pull low and would be read as a closure of the Slam Tilt switch.

Actually, looking closely at your matrix, switch 44 may show up as a single switch multiple closure, since its closure, combined with SW24 your always closed switch and SW41 (mine popper) which will always be closed if the opto is unblocked, would cause a slam tilt if SW44 had a bad diode.

What can we rule out then...
You can rule out every switch in columns 1 and 8 as there are now switches in row 1 that could pull the row low. You can also rule out all switches in rows 3 and 5 since there is no way to pull these rows low during column 2 time to activate any other column switches.

Great suggestions by pin_guy. If you're still having trouble identifying the pattern of closed switches I would highly recommend the following product. I beta tested it and it works fantastic for finding the cause of phantom switch closures.

https://pinside.com/pinball/forum/topic/interest-check-wpc-diagnostic-tool-playfield-switch-diode-issues#post-3330736

Problem may not be in the same row.

It takes three closed switches to cause a phantom switch closure. What other two switches are closed when this happens? One will be in row 1 an one in row 7.

This is a lot more work than necessary, and like Zaza mentioned, you only have a 66% chance of fixing the problem by doing so. It will be much easier to test these diodes rather than replace them.

The diode itself is connected across the switches common (no wire), and normally closed (white wire) contacts, to test the diode, you will need to hook up your meter across the diode then close the switch, you should see a state change from shorted ( at or near 0 ohms) to an open (2M ohms +) after closing the switch; if you still read a short, or very low resistance with the switch closed, you found your problem.

Updated list of possible bad switch diodes based on the new information ... you are down to 6!

ccc matrix 2 (resized).jpg

While you're checking the diodes, visually inspect them and make sure a lead isn't shorted to ground or another lead/lug.