Some possibilities:

I had this problem with PCs that antagonized everything in the world whatsoever and got no allies. I realized it was directly correlated with the perception that, although I was not railroading them, I was GMing an adventure in railroad format (read: 99% of adventures in the market made after the release of dnd 3.5). In railroad format, the encounters are carefully balanced to always be winnable. The PCs act brazen simply because they know they’ll get away with it every time. Even if the consequences are nonphysical (you fought the Prince of Mendev, now the guards are hunting you), they know they’ll brush those consequences easily as well because the GM’s plot must continue. Which brings me to…

Railroad is the ugly word. No one will ever admit they are railroading and everyone has a different version of what “railroad” means. I recommend some reading of The Alexandrian blog and bankuei’s blog. They have the pretty radical definition of raiload. For both, railroad is when you run a prewritten plot. Alexander writes this in a very gentle manner like a pre-school teacher, Bankuei will accuse you to your face like a drill instructor.

I don’t agree with their definition of railroad, but I recommend you to read both nevertheless. I had this same problem, as I said before, and this was the solution: Stop running pre-written plots, as they were the reason for 80% of my woes in tables (the other 20% are dealing with lost players who don’t know how to play without being railroaded and players who think the GM is their fucking personal jester instead of another player with their own opinions and boundaries).