Summertime conditions in the Rockies constitute a recipe for thunderstorms.
The ingredients are strong sunshine coming at a direct angle to warm the land below, sharp differences in temperature between air warmed by the mountains and cooler air over adjacent plains, and enough water in the air. Thunderstorms owe their existence to thermals, or localized pockets of warm air that rise, and the topography of the Rockies is predisposed to create intense thermals.
The sun heats a mountain chain averaging 8,000 feet in elevation much more than it heats the low-lying plains, and the mountains in turn heat the air. The magnitude of the updraft is directly proportional to the intensity of the heating, and the summer sun is the hottest.
When the resulting warm bubble of air accelerates upward because of its pressure difference, it can cool to the point that the water vapor within it condenses, and if there is enough, precipitation occurs.
It so happens that summer airstream patterns bring in plenty of moist air from both the Gulf of Mexico and the Gulf of California; at other times of the year, there is more dry air from the continent. The outcome is a cloud, rain, and a ruined picnic.