Cervine road rage
Both the truck driver and Mr Hippo should be glad it was a deer and not an elk (moose to the North Americans). Those bad boys are enormous when fully grown, with legs that keep the body clear of the top of the bonnet (hood to North Americans) on most cars and small trucks. This means that when you strike them, the mass of the body comes flying across the bonnet, through the windscreen (windshield to North Americans) and slap bang into the vehicles unfortunate occupants.
Add this to common elk features like being obstinate, not very smart and particularly fond of standing in the middle of darkened roads, and you can see why so many people are killed in collisions with them in Scandinavia. Oh yes, and they are often drunk as well, as the plant matter they munch on can ferment in their bovine style stomachs, making them more stupid than you'd expect.