Well it could have been heat like you said, but that is probably not all that likely because it would require a whole lot of heat... Maybe if it was left in a damp environment causing corrosion... Or possibly the metal was contaminated before it was coated compromising the material... This happened with aluminum used on Ford cars in the 2005ish models... The aluminum was contaminated with iron and it oxidized under the factory paint, causing the finish to bubble off. It looked like rust forming under the paint even though aluminum can't rust. It was a chemical reaction.
The most likely scenario is the most common issue with any of these bridges causing finish corrison/erosion is when they are not wiped down after use. That bridge has the patent text so it could be as old as an original lo pro... Which would place it as early as early 90's. Though that looks awful, someone would have to have some seriously acidic hands to cause that. You can see it's only on the side of the bridge there the trem bar isn't... It's where someone's hand would rest. Either way, the whole trem is done... Time for a replacement.