maybe a close up photo would help - I can think of three things.
1. rodent damage - mice love to chew on stuff like that. but the chewed area looks smaller/finer than a rodent could do
2. insect damage - but I don't know what kind of insect could/would chew up that kind of synthetic material
3. chemical damage - I wonder if some kind of solvent spilled on it or something that dissolved the material.
actually, I'm not sure what that handle is made out of - is it synthetic or wood or bone? if wood or bone, then most likely insect damage.
Flint....I have the identical 145 and unbelievebly it "melted" for the want of a better word in exactly the same location below the cross guard..I was gutted when I first saw it.....only time I have ever seen it happen with all my fixed blades <about 1000 at one stage>.apart from Loveless below.....I put tape round the large gap and intended it to be a user...never have used it..and it remains in near mint condition apart from the now taped gap.....I reckon you can add a 4th possibility <certainty as far as I'm concerned> the handle material itself is at fault....must be more examples...as soon as I saw your photo I realized what you were going to say....I'll dig it out if needed for further investigation/photos....Schrade, anything was possible...it's well documented here that my absolute Mint Loveless which had never been handled by any anyone started to disintegrate near the cross guard...Hermann Williams kindly removed it and replaced with Ram Horn handles...Hoo Roo