The two parts don't "gas up" until they are mixed. Maybe it's the change in pressure, but methinks it's more likely that you didn't have the cap screwed on tightly.

Next time, get Micropur instead -- one tablet per liter (the tablets are easy to break up for treating smaller water volumes). Identical active ingredients, but in a more potent (concentrated) form. No need to count and mix drops, no five-minute solution mixing wait, and no more worries about leaks or spills.