Glass damage is covered under the comprehensive portion of an auto insurance policy which if you only have liability and not physical damage coverages of comp and collision then you would not be covered for side window to be replaced by insurance.

Broken windshields and other glass repairs or replacements are typically covered under the comprehensive coverage portion of an auto insurance policy. Comprehensive coverage normally provides coverage for physical damage to your vehicle caused not by a collision with an object or another vehicle, but by a variety of other specific perils.

If you did have comprehensive coverages then you would have to pay a deductible to get the side glass replaced. There are some states that waive the deductible for a comprehensive claim for glass repair or replacement, in some states such as Florida it is only waived for front windshield claims.

So with only liability insurance you would not be covered by your auto insurer for the side glass that was broken due to the pop up at the baseball game and thus would be your responsibility or the responsibility of the party that caused the damage.