Straddling chamfered edge speed cushions is the most likely cause of this damage. If your tyres had blown at 70mph, the health and safety police who had them installed could have cost you your life, as they have cost the lives of at least 150 people from 2007 to 2009. Standard dimensions of speed cushion are 1,900 – 2,000mm wide, with flat tops 1,200 – 1,300mm wide leaving chamfered edges of 300 - 400mm each side. The only way all cars I know of (apart from Hummer H1s) can straddle them is with their tyres on the chamfered slopes and that inevitably leads to tyre damage. I’m hoping to set up a test to establish how many speed cushions can be straddled before tyre shoulders start to disintegrate.