I don't know if the "trilled
Norman R" is the real explanation, but there were many "rhyming nicknames" used in medieval England and it was common for names starting with R to have rhyming nicknames that began with D. Not only was
Dick short for
Richard, but Dob was used for
Robert and Dodge was used for
Roger. "H" was also used to form such names, so Hick,
Hob, and Hodge were also found. These all led to surnames -- for examples Dix, Hickson, Dobbs, Hobson, Dodge, Hodges, etc.
I don't know why the "take off hard" part doesn't make sense to you, because it is super-common to create short forms of names by lopping off the later syllables.
Al fron
Alan or
Albert,
Ben from
Benjamin, etc., etc.