In the past, it has been presumed that the main-gap breakdown in a trigatron is caused by a precursor spark channel occurring either at the trigger gap or between a trigger electrode and oppositely located main electrode. Based on the detailed analysis of voltage and current waveforms at the trigger gaps of single- and double-mode gas trigatrons, the authors have proposed an interaction between the main gap and the trigger gap under favourable conditions. This interaction gives rise to two or more auxiliary spark channels which develop between different over-volted electrode pairs in a predictable manner. The enhanced UV illumination and electric field distortion as a result of these multiple auxiliary arcs lead to the development of a streamer breakdown between the main electrodes.