Inner tube popped on back side of the valve - why? One hour after my first 20-minute ride with new inner tube it exploded with "ps-BAH!" sound. Also, as you can see on the photo, two bulges appeared near the hole. The tire (25mm) allows up to 8 bar (116 psi) , I inflated it to around 7.5 (108 psi). Do you think it's simply a production defect of the tube?

EDIT: it's not a long cut on the photo, it's a seam. See full res image
What that looks like to me is that you had the tube pinched (folded) up along side the value stem.  That would explain both the cut and the bulges.  That cut looks like a tear - a faulty tube would have more of a clean split.

When you mount tire make sure the valve is free (you can push it in).