260 knots CAS (more or less the same as IAS) is just under M0.4 at sea level.
260knots CAS at 18,000ft, say, is about M0.55
225 knots CAS at 25,000ft is also about M0.55
So if you had two sets of limits - one that said "thou shalt not exceed 260 knots CAS", another that said "thou shalt not exceed M0.55", then up to 18,000ft the first one would apply, and would 'mask' the M0.55 limit, but above 18,000ft you'd hit the M0.55 limit first. So as you get higher than that, you have to keep cutting back your IAS (CAS) to respect the Mach number limit.
As to why there's a M0.55 limit (in this case) - a common reason for a Mach number limit on turboprops is to protect against sonic tip speeds; if you do the vector addition of the tip speed at max rpm with the forward speed you'll get a pretty high subsonic speed. (There's been discussion elsewhere on the specific limits for specific turboprops...)