Why did my car try to start on its own

Hi from uk from what you say happened sorry but it sounds like the lead from battery is connected to the wrong solenoid terminal go back to car and remove earth lead from battery !! now on the solenoid there aretwo terminal postswith nuts on them ? and also a small terminal either spade or push on post terminal this terminal has the thin wire attached ? this is the key start energizer which energises starter solenoid when key is turned ?? ok? now on one of the larger terminals with nuts there will be a either a braided or copper strip already fitted and attaches to actual starter ? forget this terminal do not put any other wire/cable to it ! now the other terminal with nut you should only have a thin cable(not a wire) with an eye let terminal ? connect this cable to terminal and also the positive + lead from battery make sure nuts secure cables ? ok ? now with ignition off reconnect the battery leads to battery in this order ! connect positive 1st and secure then connect negative /earth cable and secure ok important advice! never remove positive cable from starter or whatever untill you have disconnected! that is why you had sparks ?? always when working on electrics remove neg lead b4 positive and replace pos b4 neg ok?? now all done? your sterer should operate on key only? hope this explains clearly? best wishes from lancashire uk

Assuming that the solenoid was installed correctly I have to say that you purchased a faulty solenoid.