Japanese maples are prone to several diseases, often related to weather and/or poor drainage or excess soil moisture. It's difficult to speculate on what may be affecting yours without seeing the trees in person or evaluating their growing conditions, but it is possible that they may be diseased. It never hurts to remove deadwood at any time of year - it should be noticeable as having a grey, dead look to it and no viable buds as compared the coloring of other, healthy branches. If you have a good garden center with plant problem diagnosis capabilities close by, you may want to take in a portion of the affected branches, preferrably with foliage intact, to see if they can identify the problem for you and recommend controls, if any. Be sure to sterilize pruners after making any cuts to prevent spreading any possible disease pathogens to healthy tissue.

Excess water could certainly be a factor. Japanese maples prefer a very well draining soil and if the soil remains overly wet or soggy for any length of time, it would pose problems eventually.

Japanese maples are susceptible to many things as stated above, but particularly verticillium root rot when one whole branch dies and the others are fine. I believe it's encouraged by alternating wet and dry soils so being on a septic field probably not a factor. Root rot fungi are everywhere, there's no avoiding them, you just have to try to avoid the conditions they prefer and that the tree doesn't. That means good drainage for japanese maples and watering in the dry season.

Vertical splits in the bark are often freeze damage, not much you can do there; usually where the winter sun (!) hits the tree while temps are below freezing. And trees with verticillium often have the dried autumn leaves hang on longer on affected branches than unaffected.

Best to take a sample to a nursery for diagnosis! we're only guessing here. I like to see the whole plant in situ myself but seeing a branch is still helpful.