fbreader? What is that app for? Is it heavy on the CPU?
I conclude that its because of undervolting the CPU. In my experience and experimentation, Running multiple apps at the same time causes more CPU load. Well an idiot knows that too, but, if undervolted, CPU is starved, so working on full load reducees the performance of the frequency causing, in your case, a non responsive touchscreen. Try cranking up the voltage, see if it works.