can someone explain why did he turn on robb stark? i thought he was his bannerman

The mistakes made by the Northern army (Edmure's failure to keep the Lannister's busy in the Riverlands as instructed, Robb breaking his marriage contract, the Karstarks going home) convinced Roose that the North couldn't win.

Tywin promised to make Boltons the new Wardens of the North after the war if he helped betray Robb.

Roose chose to go with the winning side.

ok i saw the part after he turned side, but never showed the reason why he did
in the book it said he was holding harrenhal with archers

Can you not make a new thread for every GoT question? If you have another question, bump your old thread. Or an actual GoT thread.

It's because Roose loves leeching, and Robb outlawed leeches in the North.
Or maybe it's what kayeto said. Either one....

lol what does it matter?

Basically this. Plus there is speculation that Roose simply doesn't give a shit about anything (I think he gives readers a more sociopathic feeling in the books) and decided to choose the side that suits his survival. There is also some history between the Bolton and Stark houses. I believe in the history of of the books the Bolton house has tried to become the Warden of the North many times over hundreds of years but always failed vs the Starks. So maybe Roose was finally able to do it with his involvement in the RW.