What is the cause of the failure of this staircase?

There are no distribution and top bars returning from the landing. That is the most likely cause of this failure in my opinion.

Wrong detailing of reinforcements. The reinforcements from the flight ought to act as top bars at landing. While those from the landing will act as top at the face of flight forming a scissors-like shape

The span is too long as a simply supported member.

The way I see it, this failure probably resulted from just the dead load of the stair. To bad!
Probable causes;
i. No top rebar to resist shearing stress especially at that end where high stress is expected.
I. Absence of distribution bars
iii.The waist might be insufficient considering the span.

What I could deduce here is wrong detailing which is not limited to: 1. Lack of distribution bars.
2. No top bars to resist shear i.e. cranking of flight bottom bars to become top bars and vice versa. Also no provision of extra bars to connect the resulted flight and landing top rebars.

# Reinforcement placement is wrong @ support.
# And may be concrete was poured in 2 different sequence with no bonding between old and new

Lack of continuity rebars at the location of failure. This has limited the effective depth resisting shear and moment at the location to the concrete cover which has triggered shear failure sInce d is too small.
A more general answer is poor detailing.

vertical shear force caused current damage. Section shear capacity is not enough what can be seen in lack of shear reinforcement around longitudal rods also.

Detailing error
No top Rebar from the beam to around 1/4 of the span of the stair.

No masonry wall at the sides to support the staircase

No Tension lap at the landing. There should be alternating bars and an extra set of bars throughout landing and half of the stairset

wrong arrangement of reinforcement