There are certainly degrees of "strange" that triggers various levels of scrutiny. I'm Norwegian. Live in England, and used to travel to California regularly on business. I never had any problems entering, the US thankfully.
But I repeatedly tried to print my boarding pass online in advance and check in using machines at the airport. In London that always went smoothly. At SFO, it almost always caused me to be referred to the ticket counters due to some undefined "problem". The staff were equally baffled every time, and the only thing they could come up with was that the system flagged me because I was travelling to England with no onward ticket and no visa (don't need one, as Norway is in the EEA which gives me the same residency rights in the UK as EU citizens) - they at least claimed that their systems did not give them any reason.
Mine was just a minor hassle, but I would be surprised if the systems "score" people based on tiny little unexpected deviations from the norm like that for various additional levels of attention. (I still printed the boarding passes every time, as whenever I couldn't check in, all I needed to do was look a bit baffled and walk over to the premium checkin and they'd get me processed very quickly)
and the only thing they could come up with was that the system flagged me because I was travelling to England with no onward ticket
This is quite likely the issue. The UK are an extreme stickler in this regard.
When you fly from Zurich, you need to show your passport to leave the Schengen part of the airport and in addition there are strict controls at the gates for UK bound flights, where your id is checked again.
Contrast this to flights within the Schengen room, where you scan your boarding pass (mobile, home printed, or airline issued) at a couple of automatic gates and you're in the plane. No id at all required.
From what I've read it's more and more the case that countries use airlines as cops to deny entry at the point of origin. If somebody with invalid travel documents, or missing visa gets onto the flight the airline can be heavily fined.
But I repeatedly tried to print my boarding pass online in advance and check in using machines at the airport. In London that always went smoothly. At SFO, it almost always caused me to be referred to the ticket counters due to some undefined "problem". The staff were equally baffled every time, and the only thing they could come up with was that the system flagged me because I was travelling to England with no onward ticket and no visa (don't need one, as Norway is in the EEA which gives me the same residency rights in the UK as EU citizens) - they at least claimed that their systems did not give them any reason.
Mine was just a minor hassle, but I would be surprised if the systems "score" people based on tiny little unexpected deviations from the norm like that for various additional levels of attention. (I still printed the boarding passes every time, as whenever I couldn't check in, all I needed to do was look a bit baffled and walk over to the premium checkin and they'd get me processed very quickly)