Hmmm...two possibilities. It is a fast growing, subtropical species. That means that if it isn't rotated frequently, it will quickly grow toward the light. Also, it needs more water than desert or semi-arid cacti, and may droop a little if kept to the same watering schedule as the "Red-Headed Irishman" next to it. If the wilting goes on more than a few days, the curl may become permanent.

Seems to be curling away from the window to me which indicates a negative tropism which would allow it to expose more of its surface area to the light at the high angle which it comes in through the window. This could be a whimsical conjecture on my part though.

Most members of the Opuntia subfamily do the exact opposite, Loosestrife, so they can minimize their exposure to the heat of the sun. This one might be an exception, since it is native to dry tropical or subtropical forests. I suspect that it has been rotated recently, but wasn't for too long before that.

May I join in please because Grannysal, because I think this might answer a problem I have with a couple of cacti, one's an Echinocactus and I think the other might be a young Echinocereus. They are both growing one sided and don't seem to respond when I turn them round. Have wondered if perhaps they are doing what Loosestrife describes? I have never had this happen with cacti before.

from a biological point plant stems show positive growth [positive phototropism] to light due to the growth hormone being in a higher concentration on the shadier side. actively moving away from the light side or being destroyed by the light.
so Grannysal have you turned the plant recently?

Thanks so much to you all. I haven't turned him.
And I had no idea his neighbour was the 'red headed irishman' and he's been mine for more than 15 years. I think the curl may now be permanent so I shall view him as quirky. A bit like me.

All of us plant freaks are viewed as a bit bent, anyway! :)

'drawn to the light side' if you don't mind tug haha

Ah! That sounds better, Sbg! Also "drawn up unto Heaven", by geotropism!

yes very good Tug :o)