Poor fitness, out of the saddle is tough, the legs wont like it till theyve been forced to do LOTS of it, and heavier riders have a tougher time with it than smaller riders. Over 200lbs it gets damn tough from what Ive seen, but those clydes that can do it are often deadly fast.
You can vary the output by using your arms to take some weight off, turn a bigger gear and do it a bit more slowly, its like climbing stairs basically. Find your own standing threshold and keep working at it, it really doesnt get much easier, just faster. Although lots of riding makes leg burn prettymuch a thing of the past. Instead, when you push too hard too long they either cramp up or just kinda give out---and that is a very disheartening weird feeling, ive had that happen before.
As a rookie messenger I was pathetic at it, my brain wanted to, my form was good, my legs protested, my heart wasnt too happy bout it neither, 5 years later Im quite good at it. I spend about 1/3rd to 1/2 my time working out of the saddle usually accelerating as fast as possible or stopping as fast as possible, took a LONG time before I could do that 5 days a week at will and not get beat up tired from it. It is one of the toughest things to do on a bike, and its often very much a sign of a bikers strength, there are exceptions of course--Basso, Ullrich for example, then there's Armstrong, he went out of the saddle like few Ive ever seen.