You may be leaning on your handlebars when you stand up, which positions you too far forward. I read somewhere that your hips shouldn't come past (in front of) the bottom bracket. Assuming your bike has been fitted to you properly, try shifting back and keeping more weight on your legs.

That said, I've been training to climb standing, so I can switch back and forth between sitting and standing. Standing for long periods is tiring!

Cheers