I have experience of pigs that have had this problem.
One recovered in a few hours with no treatment, I suspect he had slept awkwardly and with exercise everything started to work as it should.
Other pigs responded to painkillers, such as Calpol, I suspect that the problem was a pulled muscle, as you say, they do like to jump the last few inches back into their box!
A pig that suddenly loses control of both back legs lacks is more of a concern. (This always seems to happen overnight, rather than during the day)
At worst, you must always suspect damage to the spinal column until proved otherwise. This is extremely unlikely to occur in a pig that has not been involved in an accident.
I have had success in treating these cases with Osteocare and Vitamin D. I have often seen an improvement within 12 hours of starting treatment.
All pigs had recovered within a few days.
It is interesting to speculate on the cause of the condition. If Osteocare and Vit. D cure the problem, it is reasonable to assume the pigs were deficient in that area of their diet. I know my pigs get a more than adequate diet, so I would question the deficiency theory. Also, why does the condition have such a rapid onset?
Malnutrion/deficiency would surely show itself by a gradual onset?
Anyway, try the Ostoecare and Vit. D supplement and keep us informed as to progress.