How can you explain this storm phenomenon?
This case is an excellent example of urban-induced convection.

Urban areas have very different surface and soil properties, leading to a different heat balance relative to more natural environments. At times when solar shortwave radiation forcing is high - summer time, early afternoons - urban areas act as heat islands. Sensible heat flux and outgoing longwave radiation are then increased relative to the environment. While latent heat fluxes are decreased because of lack of soil moisture and vegetation in the cities, there is enough moisture coming from the environment to trigger convective cloud formation and rainfall downwind from the urban area.

As to the question why did this occur in four urban areas at the seemingly similar time, it just happened so. 

See this review article on urban-induced convection and associated physical processes for more details.