The testis develops in the abdomen, somewhere near the kidneys, then follows a path to ‘travel” down into the scrotum as the child grows.
It may be located anywhere along its normal path of descent into the scrotum. Every newborn male should be duly examined after birth to check if the testes are in the scrotum.If a testis or testes (both) are not in the scrotum at birth , they may still descend later but if still undescended by age 1year,it is not likely to descend again.

This has to be examined by a Paediatric Surgeon ,because a swollen scrotum may be a hernia or a hydrocele.Treatment(surgery) is similar but the timing is different. A hernia may need to be repaired earlier to prevent complications.
There is really no way to prevent it occurring again.
Hydroceles are not hereditary but a small percentage occur in some families, especially among male siblings and twins.