Multiple animal balloons are rushing toward your balloon. If they are a different color from your balloon they will shrink your ballon and eventually leave it deflated. Animal balloons of the same color will make your balloon grow if they hit it. Tap the wrong colored balloons to prevent them from hitting yours to make it to the next level.
But beware, those pesky snake balloons don't care what color they are, they will shrink your balloon regardless! Be quick and attentive to get your balloon to size 100 before time runs out.