What causes Mach No. to increase with altitude at a constant cost index?
Cost index is a function of Time Cost/Fuel Cost and is more related to True Air Speed than Mach Speed since Mach Speed varies with altitude.

At FL300 M.85 equals 501 knots but at FL400 M.85 is only 487 knots. In order to maintain 501 knots at FL400 you need to increase your Mach speed to M.87