How does Richard become Dick?

[aenea]

And what about Robert turning into a Bob? Where did these nick-names start?

Nicknames seem to usually have most of the sound or part of the actual name to them. Why do some names just not follow this pattern, and where did the change come from?

[Ice_Wolf]

This page could help you.

According to the Oxford Dictionary of English Christian Names pet-names and diminuitives were formed in a variety of ways.

One popular method was substitution of consonants, with apocope (dropping afinal letter). Thus you have Harry = Hal, Mary = Mally, Mall. Then the nicknames were changed a little bit. Mally = Molly. (The earliest nicknames for Mary were the diminuitives Marion, Mariel, and Mariot; Mally was at first much commoner than Molly but died out at the end of the 18th century in England).

Another process then subjected names or nicknames to rhyming:
Ricard = Rick = Dick = Hick
Richard = Hitch
Robert = Rob = Bob = Hob
Margaret = Mag = Meg = Peg
Mary = Mally = Molly = Polly

[Gatopescado]

drinks too much?

[fandango]

First he don’t pay his half of the rent. Then he leaves his dirty dishes in the sink. Next he keeps you up all night playing his guitar. Finally you catch Richard, that good-for-nothing dick, sleeping with your girlfriend. Tada!