Need to be clear that the problem on the THY aircraft was with the RADIO Altimeter and not the BAROMETRIC Altimeter . I keep seeing 'Altimeter' used and there is a clear destinction between the two. The Radio Altimeter sends out a pulses of radio energy and measures the time it takes them to return to determine absolute hight above the terrain directly below the aircraft. The Radio Altimeter typically 'comes alive' around 2500 ft above terrain.(actually some are operating before this but may not be displayed to the crew -depends on the aircraft and the radio altimeter)
From experience I've seen 'ramping' of radio altimeters over the years ( This is where the reading rapidly changes to zero when the aircraft is well above any terrain) , there can be a number of causes , but moisture ingress into the antenna or the coaxial cables was a common problem . This tended to be made worse by the fact that the RA's are mounted at the bottom of the fuselage where liquids collect. These ramping events would cause false ground proximity warnings, because it would appear to the GPWC that the aircraft was descending rapidly toward the ground