http://www.uesp.net/wiki/Lore:Dragons
"It has been suggested that they may be able to shapeshift in order to disguise themselves. It is rumored that Tosh Raka, leader of the Tiger people of Ka Po' Tun, has succeeded in transforming himself into a dragon, a rumor which is substantiated by Mysterious Akavir"
so once again here's the reasoning:
1- in elder scrolls lore dragons are rumored to be able to shapeshift
2- most dragon abilities stem from dragon shouts
3- as a dragonborn, you can learn dragon shouts
4- ???
5- profit
if there's anything that contradicts the lore in this logic please by all means, point it out.
There is nothing. Actually, shapeshifting is not something special in TES universe. You can shapeshift into a werevolf. Vampires are not the same, as there you do not shapeshift back and forth, but is transformed once (semi-irreversibly). The point is to find the right necessary conditions needed for shapeshifting, such as certain blood, or words (shouts), or blessings or curses of Gods, etc.