No explanation offered in the original paper On Hortalotarsus skirtopodus, a new saurischian fossil from Barkly East, Cape Colony, though the author does indicate it was all about the distinctive Tarsus (skeleton) (back of the foot).
Nothing in Massospondylus, the accepted family name for the dinosaur.
The Spanish Wikipedia on Hortalotarsus offers “tarsus of a young bird”. Ancient Greek had a word ortalis (no h) meaning “fowl”, and its diminutive ortalichos meant “chick, young bird”.
Which means that if “tarsus of a young bird” was what Seeley was going for, it should have been ortalichotarsus.