Why does horseradish paste "curdle" when cooked? I once made a tuna marinade that included some imitation wasabi paste. Usually when I cook like this, once the fish is done I cook down the marinade into a thicker sauce. In this case, the sauce quickly clumps when heated into a goop with a similar appearance and texture of cooked chicken fat. Obviously then I can't use it, so since then I just throw out this marinade since I can't use it without heating it (it had the raw fish in it.)

Why does horseradish do this? I'm assuming that's the culprit ingredient since it's the only unusual thing that I don't have in others that don't have this problem. And in any case, is there something I can do about it, or do I just have to throw this one away?
Not sure what else is in your marinade that could be reacting to the horseradish, but since it's an imitation wasabi paste it may have things like corn starch or arrow root in it which will clump if not mixed well and then heated. You might want to just try whisking the heck out of it while it's cooking or using an immersion blender on it when it's done.