Basically down to staff shortages as well as sickness (happened last night as well). Simply not enough people available to cover the required legal fatigue breaks. Tower bandboxed on to the one frequency (118.5), one person doing departures and arrivals, which is why the arrivals spacing had to be increased to 5 miles during peak periods and therefore inbound delays.
I think you might find that this will happen more often over the next few months whilst staff levels are low at heathrow.