Well, Shandy is that blend of beer and fizzy lemonade - could be a link! And some or most Americans would use a -d- sound for the -t- spelling in shanty - the song or rhythmical chant of sailors.
Possibly, also, these people's parents liked the sound of
Mandy (from
Amanda) but were deterred by its popularity at the time so just tweaked the spelling, splicing on the -sh- sound they would have known from
Michelle,
Cheryl and such.
Apparently, quite a lot of black American parents have named their daughters
Brandy or
Brandi or Brandee, presumably in a quest for soemthing strong, brown and intoxicating. (Haven't met any myself, but black Americans are thin on the ground here.) Shandy could fit in with that naming habit quite comfortably too.