Government report has been released:
http://www.bst-tsb.gc.ca/eng/rapports-reports/aviation/2021/A21Q0052/A21Q0052.html?utm_source=BenchmarkEmail&utm_campaign=Aviator_Newsletter_-_411_-_Mar19&utm_medium=email
Basically a communications breakdown and lack of clarity over safe procedures.
The person on the grass mower may have started moving next to the runway just after the Twin Otter landed, not realizing that another plane was about to land. The field is basically used only for skydiving, but occasionally a small plane will fly into the PPR (Prior Permission Required) airport. The person flying in there in his personal aircraft was one of the skydiving company's pilots, and when I jumped there years back I have seen one of the owners of the company fly in with their own little Cessna.
The person cutting the grass wasn't one of the usual grass cutters, and had done the task only a few times. The grass cutters had been informed they could cut grass when the Twin Otter was on the ground. The report didn't get into what else the person knew -- Whether they thought that ONLY the Twin Otter would be flying, or whether they knew that 'very occasionally there are other airplanes around -- it is a live runway -- so always keep your eyes peeled'.
The report I thought unfairly criticized the pilot for not getting prior permission to land. Yet he had gotten in communication with the Twin Otter to sequence their arrivals, so he had some permission from the current DZ traffic to land. The report seems to imply that PPR is something that one needs to get for every arrival, but isn't clear about it. However, the pilot had flown his own plane in and out of the strip a number of times, and I expect he thought he had standing permission to drop in. (I have also operated that way elsewhere, if the airport owner gave me standing permission to drop by.)
There is no mention in the report whether anyone was monitoring or usually monitors the air frequency on the ground. The grass cutter did not have a radio. Which of course would increase situational awareness.
My impression is that the grass cutters weren't well briefed or practiced on the airfield's procedures -- especially that there might occasionally be aircraft other than the Twin Otter. Lack of a radio (and one usable while running a noisy mower) certainly makes any ground activity next to a runway more dangerous.