Reminder: there are multiple reasons why IRCC might refer a PR card renewal to Secondary Review. Perhaps the most common reasons are indications the PR is abroad, there are indications some information in the application is inaccurate, or there is some indication of misrepresentation made, or even possibly made, at an earlier time. Thus, for example, if the PR used a consultant in the past who has, in some way, been connected to fraud or a significant number of clients implicated with fraud, that PR also is likely to investigated, including an investigation covering past transactions with CIC or IRCC or CBSA.
My impression is that applicants who clearly fall well short of meeting the PR Residency Obligation are indeed often if not usually sent a request for additional documents. But it is also apparent some are not, some just relegated to the mire of lengthy SR. It may be that IRCC has somehow identified the latter as likely to leave Canada even if currently in Canada.
The thing to keep in mind, contrary to what many claim or even believe, the decision to refer the case to SR is almost always criteria-driven. There is a reason. Dismiss the process as random or arbitrary at one's peril.
The good news is that a number of people have reported getting their PR cards in a matter of months despite being referred for SR. The bad news is that many others still report they are still waiting, wallowing in the mire for nearly a year, some even longer. Here too, despite the complaints to the contrary, whose case is resolved more quickly, and whose takes a lot, lot longer, is usually tied to the facts in the case and the merits . . . or, for those on the longer and deeper side of the process, the issues and concerns. Despite the denials, there is a reason. Not always, of course, but almost always.
Main thing to be aware of: for anyone who is cutting it at all close, it is not a good idea to be outside Canada while the application is in SR.