Yeah that board is a hot mess and the corrosion is probably the issue and it will only get worse. Consider replacing it, you can them for like $200. I'd just replace it for an easy and quick solution, and it's relatively cheap for what you get.
https://ksarcade.net/rottendog-security-cpu-a-17651-mpusec.html
If it weren't for the board my suggestion would be check every switch in the slam tilt column/row as it's probably shorting.
That probably IS the reason, except that the short is in the board not the playfield. Either way that board needs repair or replacement.
Don't be annoyed or upset, it's not the end of the world it's just pinball and this is what pinball does. I've lost count of the 90s games I've gotten in with a board that looks just like that. If I'm not buying from a collector, I expect it now. And even if I am buying from a collector it's still like 50/50.