It's been some time since I last operated one but SOP on our 73's was to clean up on vacating the runway, normally signalled by stowing the speedbrake. For an RTO and/or runway excursion, the flaps were run fully *out* to aid those using the overwing exits, should an evacuation be required.
Looking at the photos, if it was just a taxi speed issue, they must have been going pretty fast or had very little braking action to get the whole aircraft off the hard surface. Grass stops you fairly quickly as the tyres sink into the ground, especially when wet...