Lack of strong light and nitrogen will do that,and best answer is more light,less 'N'. And what I do if that happens,usually in the winter months,is wrap wide clear Scotch tape around the area that is at the bending spot,first putting a natural 'U' shape back into the leaf.
In time,the leaf will develop strength,and the tape can be removed,and you will find that the leaf regained its ability to stay erect at that point.
Shultz Co. sells a Cactus liquid fertilizer with low N.
2-7-7,and that would help during the low light winter months.
Bob