A picture would be nice...

Unfortunately, Alan frames do often fail at the lugs. Most often the head lugs, but sometimes the seat lug as well, as you discovered. Do the cracks extend all the way through the lug and the seat tube? Drilling a stop hole at the end of the crack can help slow its progression, but it has to be at the end of the crack to be at all effective. You may need to use a dye kit to determine this. Keep in mind that unlike steel, aluminum can fail very quickly and with minimal warning.