No math is involved, only common sense : there's a certain amount of natural space debris surrounding planet earth. This amount has reached a steady state over the past few millions or billions of years. Over the past few decades we've started launching stuff into orbit. Some of that stuff explodes. Hence, there is now more debris surrounding planet earth.

Whether or not the amount of extra space debris has already caused an increase in collisions is a different matter, for which some statistics would be required. But that wasn't the point I was making.

In fact, it's quite the opposite of the point I was making : something has hit the ISS, and whether it's man-made or natural is (AFAIK) not specified at this time.