I thought it meant the same as earworm - a tune that gets stuck in your head, but the Concise Oxford Music Dictionary gives support to MGM's view:

maggot. Old Eng. word meaning 'fanciful idea', used by 16th- and 17th-cent. composers in titles of instr. pieces, often country dances, e.g. 'My Lady Winwoods Maggot'. Revived in 20th-cent. by Peter Maxwell Davies in his Miss Donnithorne's Maggot.

LFF