Re: Explanations are due ...
El Reg has covered this previously. Capita certainly can't be blamed entirely (a large part of the issue seems to be the MoD not giving proper requirements or oversight), but issues with the system were so bad that the RAF had to stop recruitment entirely at one point until bugs could be fixed, while at other times recruits were unable to sign up, didn't receive necessary documents in time to start training, and even in the best case mostly took over a year to actually process applications.
Note that the new system went live in 2017. Most of the systemic issues you mention might contribute to a gradual decline, but are unlikely to have resulted in a sudden drop at that time. I'm not aware of any major events towards the end of 2017 that would cast the armed forces in a particularly bad light, but having recruitment drop off exactly when the entire recruitment system is shut down because it's utterly buggered is a bit suggestive.