First of all, there is no auto-cropping in Capture One.
However, sometimes you might see some cropping applied by default. The reason for this is the Lens correction settings.
By default, Capture One applies a distortion correction to some images. The thing is that Distortion tool modifies your image and thus affects the cropping.
You can easily turn it off by clicking “…” icon and choosing “Disable Default Lens Correction”.