The cause is always destruction of soft and hard periodontal tissues by
bacteria. However, the rate and extent of destruction will generally be
complicated by several factors (immune response, smoking, oral hygiene,
general health, possible hereditary factors etc).

Bacteria in the biofilm on the teeth and the body's immune response to
the bacteria cause periodontitis

The Mayo clinic has a good review of the disease

http://www.mayoclinic.com/health/periodontitis/DS00369/DSECTION=3

"Periodontitis begins with plaque. This invisible, sticky film forms
on your teeth when starches and sugars in food interact with bacteria
normally found in your mouth. Although you remove plaque every time
you brush your teeth, it reforms quickly, usually within 24 hours.

"Plaque that stays on your teeth longer than two or three days can
harden under your gumline into tartar (calculus), a white substance
that makes plaque more difficult to remove and that acts as a
reservoir for bacteria. Unfortunately, brushing and flossing can't
eliminate tartar — only a professional cleaning can remove it.

"The longer plaque and tartar remain on your teeth, the more damage
they can do. Initially, they may simply irritate and inflame the
gingiva, the part of your gum around the base of your teeth. This is
gingivitis, the mildest form of periodontal disease. But ongoing
inflammation eventually causes pockets to develop between your gums
and teeth that fill with plaque, tartar and bacteria. In time, the
pockets become deeper and more bacteria accumulate, eventually
advancing under your gum tissue. These deep infections cause a loss of
tissue and bone. If too much bone is destroyed, you may lose one or
more teeth.

"Although the destructive cycle that starts with the accumulation of
plaque is the most common cause of periodontal disease, a number of
other factors can contribute to or aggravate the condition.