Just a guess, but it’s probably a combination of two things. First, if we say a self driving car is going to hit an edge case it can’t resolve once in every, say, 100,000 miles, the number of Tesla’s and other self driving cars on the roads now means more miles driven more frequently which means those edge cases are going to occur more frequently. Second, people are becoming over reliant on self driving - they are (incorrectly ) trusting it more and paying less attention, meaning less chance of human intervention when those edge cases occur. So probably the self driving is overall better, but the number of accidents overall is increasing.