Canada

I speculate that this is the fault of the United States. The scholar, a US citizen, is entering from the US, and as per the article CBSA has concerns regarding "national security" - we know that the CBSA and the US CBP work closely together, so if some top level US official ordered the CBP to pass the "national security threat" along, that'd explain CBSA's behaviour.

Specifically, why they were concerned in the first place (because they had to take the tip from the US gov't seriously for fear that they'd lose cooperation on actual important matters if they didn't), why it took so long before the scholars were released (because they needed those four hours to make sure that they covered all their bases in case the CBP or another part of the US gov't came knocking again and asking why the scholars weren't arrested or detained), and also why CBSA can't more clearly explain why the scholars were targeted in the first place (because the working arrangements between the CBP and the CBSA, and between the security departments of the governments of Canada and the US more generally, require a considerable level of secrecy).

The article also mentions,

Kanji said that prompted them to reach out to different high-ranking officials in an attempt to get Falk and his wife released.

I like to think though that this wasn't strictly necessary and that CBSA are the good guys who would have still done the right thing in the end, after of course thoroughly documenting the obvious - why the US's tip was off and there actually was no real threat.