What is OEB48? Well basically it's an Operations Engineering Bulletin that was issued by Airbus to all operators to cover the possibility of all AoA probes becoming "blocked" which could then cause the aircraft to go into unwanted protections.
The reason αprot decreases with mach number is due to two things, compressibility effect and critical mach number. Above about Mach 0.6, Calibrated Air Speed and Equivalent Air Speed diverge due to compressibility effects, meaning CAS over-reads compared to EAS. Airbus and most other aircraft PFDs show CAS on the airspeed tape, so at higher Mach numbers the Vls, aProt and aMax displayed must increase to compensate for the growing difference between EAS and CAS. The other effect is caused by increasing the Mach number into the transonic range. This eventually causes small shockwaves to form on the wing, which grow in size as the flight Mach number and/or AOA increase. These shockwaves disturb the airflow behind them, reducing the lift of the wing compared to subsonic flight conditions for the same AOA. The relationship between CL and AOA are adjusted to compensate. The effect is the stall equivalent airspeed increases because the shockwaves cause CLmax to decrease.
Finally, the shockwaves can also cause the wing to stall at a lower AOA, but this is all dealt with through reduced CLMax and thus higher stall speed.
CAS Link https://en.wikipedia.org/wiki/Calibrated_airspeed
EAS link https://en.wikipedia.org/wiki/Equivalent_airspeed
Critical Mach Number link https://en.wikipedia.org/wiki/Critical_Mach_number
Accident interim report for Lufthansa Accident http://www.bfu-web.de/EN/Publications/Interim_Reports/IR2014/I1_Report_14_6X014_A321_Pamplona.pdf?__blob=publicationFile