Dogs bark as a form of communication — as you no doubt know.
The adult wild canine will howl or bark as a way to signal its presence, location, and condition to others of the same or foreign packs. Additionally, canine pups will bark for attention from their mothers.
Domesticated dogs exhibit behavior which has been described as a permanent state of quasi-adolescence. The process of domestication has filtered in preference for traits which would be seen as immature in an undomesticated canine.
Seeing as how the morning barking is a recent occurance, you should look to more recent events for the cause of their behavior.
Did their environment change recently?
Are there noises, which may be almost inaudible to you, that would trigger a barking chain?
Has your routine changed in some way? Does it appear to them that you are unavailable?
Were they being housed in crates since prior to the recent spate of barking?
Is there some other need that would prompt the dogs to bark for attention?
Do they lack food or water in the mornings? Do they need to “do their business”?
Not all dogs will bark for such needs, of course. I've seen that smaller or confined dogs tend to be more likely to do so.
In your situation, of course, the dogs are barking in the mornings and not at night when they are first secured in the crates. That leads me to suspect the first or the third of my presented possibilities.
If it is at all possible, you could try releasing the dogs for a few mornings. Do they bark?
If they need briefly let outside, then it might be necessary to do so at about the time when they'd begin barking. Whether you adjust your schedule or are able to get some sleep later is, of course, something you need to consider.