The questions asked in the intial letter dictated who got the first interviews/sims and who the HR department considered most suitable. Hence the highest proportion of 'passes' was expected at the start and then to gradually decline. From what I know the pass rates have been quite high but still many falling at the sim stage and that proportion of failures is also expected to increase.
Thus, irrespective of the number of applicants, the number of 'suitable' (great HR word!) successful applicants is expected to decrease over time so to keep options open and the standards to the required level the taps have been opened again.
It's a tough world but, at the moment, the airline can pick and choose.
Good luck.