If you're on the periphery of the 3G signal in your area, then something as benign as turning the device horizontal certainly could cause it to drop off to a stronger 2G or 1G signal, even having your hand block the receiving antenna would make a difference.
If you're not, that is, if you're in a strong area for 3G and this is happening, your device needs to be looked at by a tech and perhaps replaced.
Check tower and signal-strength maps for your carrier in your area.