Ethylene and Ethyne (accetylene) have a unique property called decomposition. The ethylene molecule will decompose into a C-C and 2 H2 molecules. This reaction is exothermic and creates 3 gas molecules for each 1 ethylene molecule. Because you have heat released and a trippleing of the pressure, it is quite explosive in nature.
The point at which a ethylene molecule decomposes has been studied and I know if you research decomposition you can get all the details. There are some charts on what temperatur and pressure this happens, but on the safe side at 800 psig we kept the temperature below 600 F and at 2000 psig under 400 F, you could draw a straight line between them. Acetylene is even worse or will decompose at lower energy levels. As JLS indicated, copper and other metals can act as a catalyst and change the the point that a decomposition occurs, so they are advoided.
Interesting too, ethylene will also spontaneously polymerize with the right catalyst at normal temperatures and pressures. A great cataylst for this is FeO (with the proper number of each). The reaction is not fast and they only side effect is cleaning out plastic from time to time.