The duo agreed to put on a united front before they announced their split on Monday (24.02.14), after almost nine years of marriage, but they began "unravelling a long time" ago and their relationship reportedly got rockier after Miley Cyrus twerked against the 'Blurred Lines' hitmaker at the MTV Video Music Awards (VMAs) last August.
A source told UsMagazine.com: "The Miley Cyrus fiasco was a big test of their relationship. He asked her to help him out as a friend and a wife, and defend him and the performance. He begged her to speak out for him and be there for him.
"She agreed to be the good wife after he begged and pleaded. Then he asked her to stick with him through awards season, and she agreed. She attended the Grammys with him as a favour."
Paula, 38, frequently voiced her support for raunchy performance and previously said: "They rehearsed for three days. Robin told me in advance. I thought it was great."
The 'Baggage Claim' star also defended Robin, 36, after he was photographed grabbing a model's butt at a VMA after-party and was accused of cheating on her.
But the insider said: "Their marriage was unravelling a long time. It is not a shock to Robin or Paula, and it is something they have discussed for a long time."
The couple, who have a three-year-old son, Julian, together, released a joint statement on Monday that read: "We will always love each other and be best friends. However, we have mutually decided to separate at this time."