I was talking with Mohamed about this earlier today - it’s something that can make you think you are off the track while in reality there is a much simpler explanation :-)
Let’s say you have this pipeline - the traditional:
pool:
vmImage: 'ubuntu-latest'
steps:
- script: echo Hello, world!
displayName: 'Run a one-line script'
This is a CI build - you have no trigger, but as per documentation you have an implicit trigger on all branches (as per documentation).
The main thing to be aware of is that this type of trigger will operate on all branches, and on all events. All events include branch creation as well, so don’t be surprised if you see a slew of pipelines running when you create a branch - chances are you are using an implicit trigger and they are running as expected!