Very much looks like the elastic threads failing, and that's very likely due to washing or tumble drying at too high a temperature. We've had the same with various stretchy jeans. Unless you want to reweave the fabric, it's not fixable I'm afraid.
Just make sure that from now on you do not tumble dry (or wash at more than 40C) anything that contains elasthane.
The care tag stitched into the jeans will tell you the same, see here for an explanation of the symbols.
The diagonal lines you see are not diagonal weave, but an optical illusion that is typical to all kinds of twill weave (wikipedia). Jeans fabric (aka denim) is typically a twill weave. So the stretch lines you see in your trousers are actually along the weft, not diagonal.
Zooming in, you'll clearly see individual strands of the fabric in the damaged area, while the strands are much, much tighter in undamaged areas. There are some possible causes that come to my mind:
The weft yarn is stretching. In contrast to the warp yarn, the weft yarn doesn't have to take much strain during the weaving process. That means that manufacturers can use lower quality material for the weft. If you wash and tumble dry your jeans regularily, that damages individual fibers in the entire garment. When the higher quality warp yarn can withstand more of this damage than the lower quality weft yarn, you get a stripey pattern everywhere the weft stretches due to the damage. If these patterns only appear on trousers of one brand but not on those of another brand, this is the most likely cause.
This fabric has an elastic component that is failing. Denim usually doesn't stretch. To create stretchy jeans, there's a tiny elastic thread woven into the fabric. This elastic thread is so tiny, you'd need a magnifier to even see it, but if it's tightly woven into the fabric, it can withstand a lot of strain. What it cannot withstand is heat, so washing and/or dry tumbling your jeans too hot can break those elastic threads. If these lines only appear on stretchy jeans but not on non-stretchy ones, this is the most probable cause.
Repeated strain. It's a strange coincidence that the stretch marks have the same width and cover several weft threads. Why would several threads in the same area fail instead of an even distribution? Possibly because these areas snag on something or are poked by something that wears them out in this specific pattern. It could happen while you wear them, or maybe in the washing machine or dryer. If these lines appear on all of your jeans, regardless of brand and quality, this is the most likely cause.