These bolts break because of fatigue cracks in the bolts. As stated above the cracks are caused by vibration. They start at a blemish or machine mark or thread at the surface of the bolt and then with each vibration they slowly eat their way across the diameter of the bolt.
If you look at the plain of the crack after a bolt has broken you will be able to see where the crack began (the origin) and then tiny microscopic lines emanating in ever larger arches as the crack progresses through the metal. (Use a microscope or powerful magnifying glass)
If the crack has sufficient time to work its way clear across the shaft of the bolt then the head of the bolt will fall off but if the crack is only partially through the bolt ... chances are that the bolt will snap when removal of a bolt is attempted.
In the case of a 100% fatigue failure the plain of the broken bolt will be smooth and dark from exposure. But in the case of a snapped bolt only a portion of the plain will be smooth and dark while the portion that just snapped will be shiny and rough.
(Former metals lab tech)
PS My advice is to remove the bolts and replace them as soon a possible because eventually the heads will simply fall off.
Soak them with penetrating oil and let them soak for a long time. Then attempt to remove them turning slightly in both on and off directions. If you feel the bolt break lose then you are home free but if it turns quietly with constant resistance then you are twisting the shaft at the crack and the bolt will snap.
1993 190E 2.3
2000 Toyota 4x4 Tundra