Being from EU with decent contact to EU parliament... EU officials really don’t need much encouragement. The passion to regulate everything burns deeply.
Like with law enforcement, it is a target rich environment. There are many more possible cases to litigate than time or personnel to do it. So cases get chosen for a variety of reasons. Public good, self promotion, lobbying by competitors, etc...
The most likely way it happened was a multitude of complaints received in workers-rights or competition institutions because that's a formal way to complain about companies doing unfair stuff, and when Amazon abuses their workforce or vendors it affects a LOT of people with the right to turn to these institutions. These institutions will default to having paperwork supporting their process and decisions as they routinely need to use the legal system to solve problems. I think it is very unlikely a competitor could orchestrate these consequences for Amazon's actions.
As the 28th largest company in the world, there are not very many targets larger than Amazon, and most of them have also been targeted by the EU over something.
