Disc brake rotors with the six bolt design are attached with M5 fasteners. To shear off the head of a new M5 bolt requires a considerable torque. The design torque for medium resistance bolt is typically 5 Nm. It ought to take at least twice that torque before it fails so abruptly. A torque of 10 Nm is roughly equivalent to the weight force of 10 kg applied at a 10 cm lever. This would require quite considerable effort when using a common hand tool (eg a L-shaped X25 hexalobular-key).
Since this happened at used bolts I should rather suspect a different reason for failure. For example, Stress corrosion cracking (SCC) or corrosion fatigue (CF).
"[SCC] is the growth of crack formation in a corrosive environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress, especially at elevated temperature." 
"[CF] is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading." 
Soft steels (ie ductile, austenitic steel) are often at risk to SCC when subject to chlorine. (Wooden truss roofs over swimmingpools are notorious for collapsing because of bolts failing due to SCC.)
While the environment in this case is not particularly corrosive, previous over-tightening of the bolts might considerably increase the susceptibility of the material to SCC. A corrosive source may be salt, which is very common on roads in high concentrations in cold climates.
Several other causes of fastener failure are discussed in an article by Hudgins et al []. It also shows and explains the surface where the fastener broke off. You might compare your fasteners with the pictures to identify typical structures associated with different failure modes. In all cases you ought to be able to see the ductile overload happened, ie the last bit holding to the head.
tl;dr A form of corrosion might have damaged the bolts such that they rupture easily. This is more likely to happen when tightening fasteners with too much torque.
There's an old German proverb that fits here: "Erst fest dann ab!".
An approximate translation is "First its tight, then broken".
: A Hudgins, B James, FASM, Avanced materials & processes, August 2014, 18-22 http://www.asminternational.org/documents/10192/20564188/amp17208p18.pdf/5cddb014-2b4a-40a1-b0f6-07b58906754d