Ik this isn't cannon but. When fiddleford went into the portal for that split second he actually had a super drawn out mega devil went down to Georgia style fiddling battle with Bill Cipher all within the span of a couple earthly minutes. He went into shock because he fiddled harder than ever before and it broke his brain. He switched to only playing banjo because of how hard he shredded he knew the earth wasn't ready for that power yet.