Why did my Ford E350 box truck start leaking transmission fluid?

So I left my truck and didn't drive it for about a week or two and 2 days ago I noticed a huge leak that came from the transmission. I'm really curious as to why it happened since I didn't drive it for these two weeks and it happened just recently. What is going on? Bad seals? Hot weather made it start seeping through? I checked the fluid levels and they look fine to me. Advice?

You say it had a "huge leak" but the fluid levels look fine to you.
and it took 2 weeks to accumulate.
i can bet the cooler lines are leaking some. They all seem to.
they may need clamps retorqued. There could have been a high fluid reading before… I don't know. Usually, a box van/truck has some add ons. I would start inspecting.

I agree with john, my f150 also leaked, it was the power steering hose, i cut a piece from front n mounted the hose tight

Could be a corroded hose or a bad gasket. Take the time to take the pan off, clean it, replace the filter and put a new gasket on it. Refill and check it out. If it's leaking from the seam of the pan it's the gasket. Either that or it's a power steering hose, my f250 just did it twice. Though it was trans but it was same fluid just from a different hose. If it's coming from where the drive shaft is it might be a seal and you'll need to take the u joints out to get to that.

It's a 2005 E350 Super duty with 183,000 miles on it.