Real Wild Boar mate in the Fall and have a litter in late winter early spring. Their gestation period is somewhere between 110 and 120 days. I have no real first-hand Observation, feral domestic Hogs are rare here but they are around. I've always assumed and read that domesticated Hogs are breed to have multiple litters a year, Which makes sense more litters more money, and a more efficient production model.
I imagine it can be weather-related, but the trigger for Wild Boar seems to be colder weather and/or shorter days.
It seems reasonable some lines of feral domestic Hog revert back to the standard over the generations. It has been hypothesized that multiple litters a year may be dominant in some lines. Most feral populations breed randomly and eventually over generations revert back to the base model or a hybrid very close to the original.

Where I hunt for hog in SC there are real wild hogs, they have multiple breedings a year. I cannot speak for Europe. All wild hogs in the southern states came initially cam from domestic animals that the explorers and settlers released.

While feral hogs can have multiple litters per year, at least here in Texas, it is generally about 3 litters every 2 years. The reason there are so many hogs isn't just because of the additional litters, but because of the high survival rate of the young, usually >70%.

maybe from the feeding

They say Corn is Hog superfood and increases their metabolism. They mature faster, breed earlier, and get generally bigger. The Wild Boar plague here can be graphed to show the rise in numbers is directly proportional to the number of acres of Corn planted for ethanol. There are some other minor factors, like a reduced number of hunters and counties closed to hunting.