Why did the tyre pop?

[raleighnut]

Looks like a glancing strike on a rock or summat similar, edge of a pothole maybe.
The carcase fibres look to be quite freshly cut and the (superficial) scrape on the rim is not on the outer of the rim but is more on the side. I'd still check the brake block position though.

 

[Sara_H]

Brake blocks were definitely ok.

Do I need a new rim?

 

[raleighnut]

Brake blocks were definitely ok.

Do I need a new rim?

No just take the rough bits off with a 'nailboard' or a bit of emery paper.

 

[Smurfy]

There's a massive jagged tear along the side wall, near to where it meets the rim. Difficult to know if that's cause or effect though.

Tyres were in good condition, not over inflated. At a complete loss really.

Brake blocks rubbing on the tyre wall?
Rough edge on the rim wore through the tyre wall?