Why some 2-methylpropene can be formed during the synthesis of alkyl halide? what is the mechanism?

I presume you are making the halide from treatment of the alcohol with either HCl or HBr ( to make the chloride or bromide respectively ). When the alcohol is treated with the strong acid , the OH group becomes protonated making it a good leaving group ( as a water )that can be displaced by the entering halide. A small amount of the protonated form may lose the water forming the free carbonium ion which immediate stabilizes by forming an alkene with loss of the proton at the # 2 carbon. Generally this is a minor product because free primary carbocations are unstable so the loss of water with simultaneous attack by halide is greatly preferred.