A geyser in a stormwater and combined sewer system is a high frequency oscillatory release of a mixture of gas and liquid that occurs at vertical shafts. Using actual formulations, it is not clear how to predict a large geyser height as observed in the field. This paper describes a new theory on the causes of geyser occurrence and presents a simplified geyser model. I hypothesize that, in a similar way to limnic eruptions (e.g., Lake Nyos), geysers in stormwater and combined sewer systems are propelled by exsolution of dissolved gases such as ammonia, sulfur dioxide, chlorine, hydrogen sulfide, carbon dioxide and methane that are present in these systems. In most cases, it is likely that the gases are below saturation pressure, however when a gas pocket enters the dropshaft, it moves liquid upwards. As the gas pocket, along some liquid, moves upward, the pressure decreases. The loss of pressure reduces the solubility of the dissolved gas in the moving liquid causing the rapid formation of bubbles, which in turn causes a rapid volume expansion. The rapid volume expansion accelerates the gas-liquid mixture (foamy water), which in turn causes more gas exsolution due to the continued decompression. The foamy water is more or less a homogenous flow. Once the homogeneous foamy water breaks the water surface, a geyser eruption is produced. The model presented herein was applied to two field test cases. The first test case is the controlled degassing of Lake Monoun in Cameroon, Africa. This degassing system configuration consists of a vertical pipe, which is similar to the dropshaft configuration in stormwater and combined sewer systems. The results of the model considering the two main gases measured in this lake (carbon dioxide and methane) were found to have a better agreement with the geyser eruption measurements compared to that obtained considering the main gas only (carbon dioxide). The second test case is a geysering event in a stormwater collection system in Minneapolis, MN. Because there were no measurements of concentrations of gases or their partial pressures for this test, the geyser calculations for this test assumed that the system contained three main gases (ammonia, sulfur dioxide and methane) and ten different combinations of initial partial pressures (or gas concentrations). The results show that various combinations of initial partial pressures (or gas concentrations) could result in geyser intensities similar or stronger than those observed in this system. It is worth mentioning that laboratory experiments performed by the author qualitatively support the theory presented in this paper. Due to safety considerations, the gas used in the experiments was carbon dioxide, while as in stormwater and combined sewer systems, it is believed that the geyser is propelled by the exsolution of a mixture of dissolved gases with large solubility coefficient such as ammonia, sulfur dioxide, chlorine, among others.