If you are a “patron” of a person or organisation, you are aiding that party in some manner. If you “patronise” a shop, you are giving it your custom. So why is it bad form to act in a “patronising” manner, i.e. condescendingly?
The only vague link I can think of is that if you are acting like someone’s patron, you are trying to make them feel you are giving them the benefit of your largesse, or that they owe you a debt, but that doesn’t really convey what it is to be patronising.
Any ideas?
The idea of patronage is feudal, originally.
The “condescending” sense of “patronise” actually preceeds the sense of “being a regular customer,” – it’s not something that came after.
To be a patron of the arts (or a particular artist,) you’re putting yourself in that protective, nurturing role. Stores themselves adopted the term as a way of flattering their customers.
Acting as though you are in that superior relationship to a peer? That’s another thing altogether. The sense is approximately the same, it’s just the context that makes it positive or negative.
Assuming that relationship is presumptious.
When the supposed inferior declares the dependant relationship themselves, it’s obsequious.
Damn British spelling conventionz. 
Quiet, you – you’re outnumbered. 
Serious (and totally off-topic) question: is the Z tile in US Scrabble still worth 10 points? If so, I hereby accuse you lot of cheating!
Actually, my (British) dictionary gives the -ize spelling first, with -ise as a variant, for almost all such verbs, but I was brought up on -ise spellings so that’s what I use.
There was time when I was traveling a lot in Britain on Business and I had an admin over there who helped me put together my presentations and make sure they got to the right people on time. It would drive me crazy when I’d put up my slides and find that she had changed all my -ize words to -ise.
-ize is kosher in many words in English spelling though, depending on their roots. There was an episode of Morse in which the Inspector penetrated someone’s disguise simply through recognising that someone with his background would never have been so illiterate as to use the spelling “realise”.
But don’t ask me for chapter and verse!
The Etymology Dictionary agrees with you, but the transition apparently happened pretty fast:
The approximate synonym “condescending” followed a similar path. First it was considered gracious and friendly for members of social elites to be “condescending”, i.e., to “come down” out of their elevated spheres and fraternize with the lumpens in a non-haughty way. (Sort of the early modern equivalent of being “somebody you could have a beer with”, I guess.) Only later did “condescending” acquire overtones of unjustified stuck-up-ness or disdain.
The word “mentoring” now covers the positive definition of this concept. The connotation of “patronizing” has become somebody acting like they’re your superior and is doing you a favor by guiding you but isn’t actually entitled to this belief.