All side and rear glass is 'tempered' so that, if broken, it will completely break into tiny beads of glass like gravel. Even if the impact is at the far end of the glass, the whole window will go.
Also, this glass is odd. It can resist a substantial blunt force impact without breaking, like someone kicking a window, but a tiny nick at the edge might break it.
I once broke a rear window when using a special tool, like a screwdriver to remove the trim. I must have nicked the edge of the glass, but so lightly, I didn't even notice, and my entire window exploded. Also, I have had friends break their rear window on a minivan simply by closing the rear hatch. Another friend watched the rear window explode on her brand-new 2-day-old Grand Prix when she closed her drivers door.
So, how can glass be so strong and so weak at the same time? As described to me, it's due to internal stresses. As glass is manufactured, it is formed when extremely hot, then cooled down. If the cooling is uneven, internal stresses develop. Tempering is supposed to relieve all the stresses, but sometimes doesn't work. Therefore, if you are unlucky to stress the glass along a pre-existing area of internal stress it may break.
This is rare, but does happen.
So, yes a bullet, or pellet or rock will break a window..... I know from personal experience. But this does not mean you were shot at. Your glass could have broken due to stress and bad luck, as described above.