Most bolted joints use the tension on the bolt to clamp the pieces together. Friction then prevents movement of the joint. Fasteners that are not tightened adequately will fail prematurely as the joint is allowed to move. This puts the bolt in shear where it will fail a ~0.6-0.5 of it's tensile strength. In the case of a sprocket on a wheel, not only does the loose joint have half it's strength, but the sprocket can get a small running start before contacting the bolts. This increases the load being generated.
Socket Head Cap Screws, SHCS, are actually stronger than grade 8 bolts of the same diameter. Grade 8 bolts have a Ultimate Tensile Strength (UTS) of 150 ksi* and proof strength of 120 ksi per SAE J429. SHCS have a UTS of 180 ksi and proof strength of 140 ksi per ASTM A574. This makes SHCS 20% stronger based upon the UTS. SHCS are also harder than grade 8 fasteners.
You can find this information and a lot more about bolted joints in the Unbrako Engineering Guide at
http://www.unbrako.com/docs/engguide.pdf. The figures quoted above are from page 71 of this guide.
*1 ksi = 1000 psi