My wife has gotten the SSSS the last 4 trips we have taken. We both have nexus but she keeps getting it.
Aug 2017 - YYZ - LAX - HNL
June 2017 - YYZ - LGA
Oct 2016 - YYZ - PHL - SJU
April 2016 - YYZ - LAS
Heading to LAS again in two weeks and wondering if she'll get it again.
There's a related thread that you should look at too:
Repeatedly getting SSSS on boarding passes when flying to the US It is argued there that redress won't help when SSSS occurs exclusively on in-bound international flights on a single carrier.
Would I be right in guessing that all of the flights that you mentioned are on AC? I also have Nexus and have had SSSS on my last two AC flights to the US. I am fully expecting it again next week. At least SSSS in YYZ preclearance isn't too bad, but I wish that I could do OLCI.