We have them here in the states, too, and most of the time they're not considered suspicious either.
As you've said, it could be that her injuries didn't match what the investigators thought they should look like in a mower roll. Or maybe the position of the throttle wasn't open, which might suggest the mower wasn't running when it tipped.
Maybe the ROPS didn't engage, which could indicate someone tampered with that safety feature. Maybe there were tire tracks that did not coincide with the scene, suggesting it was staged. It's winter; maybe the battery was discharged, suggesting the mower wasn't operable.
There are several things I can think of that would indicate it wasn't an ordinary accident, but I'm leaning toward it being so challenging to successfully stage that type of death that a killer would be unlikely to choose that.
And, given winter and the dead of night, it seems like a very unlikely choice for staging a death. But, that's just me.
I wish we'd get some updates.