It's a reference to Keegan Deane's character.
In British English, the word 'rake' is used to describe someone who leads an immoral lifestyle.
From Cambridge Dictionaries Online, Macmillan Dictionary and Wikipedia:
a man, especially one who is rich or with a high social position, who
lives in an immoral way, especially having sex with a lot of women
OLD-FASHIONED a man who behaves in an immoral way, for example by
having sexual relationships with a lot of women
A rake, short for rakehell (analogous to "hellraiser"), is a historic
term applied to a man who is habituated to immoral conduct,
particularly womanising. Often a rake was also prodigal who wasted his
(usually inherited) fortune on gambling, wine, women and song,
incurring lavish debts in the process. Comparable terms are
"libertine" and "debauchee".
The show is a remake of a popular Australian series and the title makes perfect sense to viewers over there. Because 'rake' and 'rakish' are words they would commonly use to describe someone like the protagonist.
Actually, the series synopsis contains a hint as to why the title exists. To quote from the U.S. version:
The series follows Keegan Deane, a criminal defense lawyer whose
personal problems and self-destructive behavior has him battling
wits and/or owing money to everyone around him, including his ex-wife,
judges, an assistant district attorney, his bookie, a brothel owner,
and the IRS.
Go over to this page for a colourful discussion on the same subject.