Apart from the sun I don't think there are sources of light you would normally encounter which could burn a shutter. I have heard stories of people having damaged sensors from shooting at laser shows, but that type of damage is different and only affected the sensor.
During an exposure the sensor has to be powered and it will build up heat by itself. What you're shooting is probably minimal. The long exposure doesn't build up anywhere near enough heat to damage itself. I've done continuous sequences of exposures of up to 8 minutes each before with no problem.
There is one more thought to the sun theory: A lot of the sun's energy is infrared. Normal lenses aren't generally well corrected into IR and in those wavelengths may have a different focus point. It may be coincidence that the difference is the same as the sensor to shutter distance for some lenses.
During an exposure the sensor has to be powered and it will build up heat by itself. What you're shooting is probably minimal. The long exposure doesn't build up anywhere near enough heat to damage itself. I've done continuous sequences of exposures of up to 8 minutes each before with no problem.
There is one more thought to the sun theory: A lot of the sun's energy is infrared. Normal lenses aren't generally well corrected into IR and in those wavelengths may have a different focus point. It may be coincidence that the difference is the same as the sensor to shutter distance for some lenses.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.