A root cause is an initiating cause of either a condition or a causal chain that leads to an outcome or effect of interest. The term denotes the earliest, most basic, 'deepest', cause for a given behavior; most often a fault.
The "earliest, most basic cause" was not parking the departure helicopter on the correct location.
If that helicopter on the ground was in the correct location, irrespective of the reason for being on the tarmac, that collision would not have happened.