My understanding of a surge is that there is a build up of pressure along the compressor stages. There are 3 different spools all compressing the air, which then enters the burner for ignition before blowing out through the 3 turbine stages that are driving their individual compressor stages. When there is an excessive build up of pressure along the compressors, valves open to relieve it. If the valves stick, or there is no other means of relieving the excessive pressure such as the speeds of the various spools being out of relation, the engine will 'cough' forwards, and generally cough up air out of the front and the back. The effect is a cannon shot 'boom' sound, with flames and sparks flying out. A spectacular sight close up at night! It is invariably quite harmless, though alarming to observe. Sometimes the engine will have to be shut down if it does not auto recover, which it usually does. There are certain parameters for different types where relighting may not be permitted.
Outside influences do not cause surges.