Good question. I worked for one of them around 2005, and my boss mentioned price scouts then. As I recall it he wasn't happy about them, equating it with cheating.
The allegations are that the price collusion really took off around 2010. We had had a few foreign companies trying to take hold, like ICA in 2003 and Lidl in 2004. Lidl faced a unified publicity attack by the big three, and didn't last long. ICA bought one of the then-larger chains, but couldn't make it work and gave up in 2015.
It wouldn't surprise me if the "big three" started working more closely together during the "Lidl invasion", and that that laid the foundations for further cooperation later.
The allegations are that the price collusion really took off around 2010. We had had a few foreign companies trying to take hold, like ICA in 2003 and Lidl in 2004. Lidl faced a unified publicity attack by the big three, and didn't last long. ICA bought one of the then-larger chains, but couldn't make it work and gave up in 2015.
It wouldn't surprise me if the "big three" started working more closely together during the "Lidl invasion", and that that laid the foundations for further cooperation later.