Speaking on British TV show Lorraine, royal family expert Russell Myers explained that Harry's bad mood was likely down to Baby Sussex keeping them awake at night.
He said: “Poor Harry, he wasn’t looking too happy. I think lack of sleep I imagine.
“Doria has gone now, Meghan’s mum, she has gone back to the US.
And the couple's reported difficulty in finding a nanny to help them with their new bub is also having an affect.
“They still haven’t found a nanny so poor Harry, he’s suffering from lack of sleep.
“But, we do know he is being quite hands on.”
Prince Philip, the Queen's husband, was missed at the event for the second year running.
The Duke of Edinburgh, who celebrated his 98th birthday on Monday, retired from official royal duties in August 2017, so wasn't required to be there.
Baby Archie also wasn't at the event, whereas Prince Louis made his first appearance at this year's Trooping the Colour, much to the delight of the watching crowds.