Been looking for reasons as to why a file could be referred for more detailed security screening. Apart from the usual red flags that are taked about on this forum (military/political/government work, country of origin, travel history, race/religion etc),
this immigration lawyer suggests the possibility of a "poison pen letter" (to IRCC) from a disgruntled ex/co-worker/neighbour etc that may accuse an applicant of all kinds of things and which could result in a lengthy investigation by security agencies. So apart from the backlog, general inefficiency and waiting on info from other countries, it would seem that there are all kinds of unsuspecting things which may cause perfectly normal and law abiding individuals to be caught up in a needlessly lengthy background check. They should just schedule an interview if they want to ask questions to clarify anything!