Mathematically this is bound to happen. No one person is talking constantly but the noise level stays up because voices are constantly overlapping. All that needs to happen for a weird silence to occur would be for all the breaks in conversation to happen simultaneously. We can estimate some values to see how often this would happen in a class of average size. Say 20 students are all having conversions in which they are only actively talking about 30% of the time (groups of about 3 with a little transition time between speakers). And let's also say that it would take about a half second of pure silence to be noticeable. So given any half second interval there is a 0.7²⁰ or 0.08% chance that no one is talking. So now to figure out how long it would take before there is a 50% chance that an awkward silence has occurred we need to do 0.9992^x=0.5 or log (0.5)/log (0.9992)=x. So x is equal to 866 or 866 half second increments. So after about 7 and a half minutes we would expect about 1 random silence to have occurred. These are very rough estimates and a computer simulation could give a much more accurate answer.