Why Sophie, not Kate, joined Prince William for film screening?

Sophie of Edinburgh left royal fans stunned with her rare appearance with nephew Prince William for a screening of documentary Rhino Man together at the renovated Battersea Power Station on Tuesday night, while Princess Kate stayed at home.

The heir to the throne was not accompanied with his wife Princess of Wales as he attended the event with his aunt Sophie in south west London.

The royals were all smiles during their unexpected outing together as the Princess of Wales and Duke of Edinburgh - William’s uncle - stayed at home.

Meanwhile, Kate's absence sparked hilarious reactions from some royal fans who began to speculate, with one asking: "Where's Kate?

Another responded as saying: "She busy in other royal engagement."

While the third one expressed surprise, saying: William appeared with an unexpected guest." 

Few others asked: "Why Sophie, not Kate, joined Prince William for film screening?"

There are also speculations that Kate Middleton is unlikely to take traditional Trooping the Colour job despite new role.

However, the Duchess of Edinburgh was "delighted to join the Prince of Wales at the event this evening to show her support for United for Wildlife"

Kensington Palace said Prince Edward's wife, who herself has a strong interest in wildlife protection, was "delighted to join the Prince of Wales at the event this evening to show her support for United for Wildlife."

Prince William’s official Twitter account said of the engagement: “This evening’s screening of Rhinoman provided a stark reminder of how wildlife rangers are putting their lives on the line every day in the critical fight to end wildlife crime.

“It is work United for Wildlife plays a crucial part in - disrupting this transnational organised crime and making it impossible for traffickers to transport, finance or profit from illegal wildlife products.”

At the event, Prince William said: "The murder of Anton Mzimba is a stark reminder of the daily dangers that all rangers face around the world protecting the natural world from the international criminal organizations that seek to profit illegally from wildlife, and I’m proud to see his life and legacy recognized in Rhino Man."

"This is not a burden they can bear alone," he continued. "

Sophie, 58, has continued with royal duties in recent weeks despite a tragic incident: the death of an 81-year-old woman who was hit by a police motorcycle escorting the Duchess of Edinburgh in London last month.

Helen Holland was struck by the escort vehicle on May 10 and was left with “multiple broken bones and massive internal injuries," which she later died from.

Buckingham Palace said in a statement to PEOPLE that Sophie is "deeply saddened" about the death. "Her Royal Highness's deepest condolences and sympathies go to all of Ms. Holland's family," the message said.