There's only one of a few possibilties here.
1. The pilots believe they're going to be released very soon
2. The tower believes they're going to be released very soon
3. Someone has told the tower or the airline, and based on what's been reported here in the US, it's airline management telling the pilots, that they can not return to the gate under any circumstances.
Why would they be told to not return to the gate?
One airline manager told a reporter that if they returned the plane to the gate, they'd have to reissue tickets, and the pilots might exceed duty hour limitations, as well as having to unload and rescreen all the baggage, so it would be cost prohibitive.
I am not saying it's right, I'm just saying that's what's going on. It's not a simple situation.