I reckon it's the tube caught underneath the tyre. It's happened to me before. The first time I thought I'd been shot.
I also had this when I was using an ancient rim and tried to get past 100psi. Got through 3 (admittedly, also old) tyres before setting for 90psi. I suspect the rim was bent somewhere and pushed into the tube at high pressure.
I reckon it's the tube caught underneath the tyre. It's happened to me before. The first time I thought I'd been shot.
I also had this when I was using an ancient rim and tried to get past 100psi. Got through 3 (admittedly, also old) tyres before setting for 90psi. I suspect the rim was bent somewhere and pushed into the tube at high pressure.