SIA Airbus's will sometimes turn east after take off and climb overhead the airport to gain some height before heading to cross the southern alps. Straight performance (or lack of) as they are normally heavy and I belive climb at reduced power so need the extra distance to ensure adequate terrain clearance.
Nothing to do with the ATC' guys, plus if you go off RWY02 there is alot of traffic coming in from the north so sometimes you will not get a turn to the west until you are above them or get radar vectored around behind them.