Refer to figure.
At sea level, the TAS is comparitively low and the speed of sound is quite high. At high altitude (for a given CAS), the TAS is much higher, because the air is thin and the local speed of sound (LSS) is less, because it is colder.
Because the Mach number is TAS divided by LSS, this means in turn that at altitude, your Mach number is higher for a given CAS, than it is at low level.

The result of this is that, as you climb, the maximum speed is controlled at low level by CAS and V_MO, and at altitude you reach your limiting Mach number M_MO , before you reach V_MO and therefore your maximum speed is controlled by M_MO.

In the descent the reverse is true. Your maximum speed is controlled by M_MO first, and then by V_MO at lower altitudes.

Alternatively, LSS increases during the descent, due to the rising temperature. Therefore, TAS must increase during descent to maintain constant Mach No.
IAS is also increasing and will eventually reach its limit. V_MO ("max operating") is the term for the maximum certified IAS.

NOTE: You can also advice the ECTM lines to show that IAS increases with constant Mach.