It is nearly impossible to give a diagnosis with so little information, but it sounds like it could be a type of root rot or verticilium wilt. It's nearly impossible to tell for sure without having it tested, but you could cut through a dead branch and see if it has odd colorations in the wood. If that is the case, one of several types of soil fungi could have been the cause.

Unfortunately there isn't much you can do for verticilium wilt, but if you got it tested you might be able to save some of the not-so-dead trees with ammonium sulfate fertilizer. Supposedly this can help with verticilium wilt, but I don't know how much it would help if the fungi has already spread throughout a significant portion of the tree.

A heavy drench of a good fungicide might help save the ones left if a type of root rot fungus is the cause. This is not a guarantee either by any means, but if you don't want to replace them you might want to check it out.

Yep, it sounds like the trees are sick or cannot transport water and minerals. You should hire a professional arborist all things considered.

Dax

Does this mean sometimes it is safer to keep a tree in a pot? I was just about to plant my first and now I wonder if I would be better off keeping it in a pot of 'clean' soil til the tree is stronger/larger? This is my first maple and I know nothing about growing them but what I have found on this forum. I especially love this particular maple and do not want to cause any harm.

Need more info age size and type of tree and soil...I don't think this post should generally scare you it is very unusually to have a bunch of large "yard " trees die like this..David