they are caused by tearing of the dermis, which over time may diminish, but will not disappear completely.
Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth or rapid weight changes. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, bodybuilding, hormone replacement therapy, etc
Either way... Any time you stretch your skin, be it with fat OR muscle, you may get stretch marks. You can try using lotion on your "expanding parts" every day to increase your skin's elasticity, but there are no guarantees it'll prevent stretch marks. :)