Over-stressing, and
string-against-string forces are inherent in the game. They're not entirely
avoidable, but you can take steps to reduce their effects. Other types of breaks
are due to racket defects or carelessness, and can be completely eliminated with
proper attention. Source
: USA Badminton
The first and most common cause of breakage is
stretching the string beyond its elastic limits-similar to pulling a rubber band
until it snaps. This usually occurs on one of the central four main strings,
within an inch or two of the racket's head.
The sweet spot is the most
resilient-and hence most powerful-area on the racquet. No matter how clever
racquet designers may be, they'll never be able to make the sweet spot extend
right to the edge of the racket head. Every time you miss-hit outside the sweet
spot, you're stressing a relatively unresilient section of the string. There's
simply not much string to stretch between the shuttlecock and the edge of the
racket, and if the forces are too great, it snaps.

Stringing at lower tension will reduce the incidence
of this problem, as will the use of a thicker string. But don't change both at
once. Approach the problem systematically, change one or the other, and play on
the new setup for a few weeks before deciding what your next move should
be.
Other relatively common causes of breakage are string-to-string
abrasion and cutting. Every time you hit the shuttlecock, the cross strings
pound against the main strings. In time, they can cut through the outer jacket
and into the core. Likewise, "cutting" and spin shots cause the strings to "saw"
back and forth against each other, with much the same effect. Again,
thick string withstands this longer than thin string, but there are better ways
to resist abrasion and cutting. Some strings have two jackets or wear layers
instead of the more common single wear layer. Naturally, two jackets will resist
abrasion longer.
Jackets may also be either twisted or braided over the core.
Braiding is the more durable approach because the over-under-over design locks
all the fibers in place. Even after a few jacket fibers have worn through, the
jacket as a whole remains intact. In contrast, a twisted jacket may start to
unwind as soon as the first fiber is breached.
A broken, split grommet may present a sharp edge that
can cut into the string. If the grommet is missing, the frame itself may cut the
string. Next time the strings are out of your racket, feel the edges of the
holes that are drilled into the frame: they may be quite sharp.
The
simple solution is to make sure the grommets are sound. A new grommet strip may
cost $5 or $6: well worthwhile if it saves you a $20 stringing job. If the
proper grommet strip is unavailable, your stringer may be able to install a
piece of plastic tubing as a temporary repair.
Temperature-related
breakage is common in cold climates in the U.S. and Europe. Cold causes the
string to contract, significantly increasing its tension. Furthermore, nylon,
which is normally a very resilient material, becomes brittle when it's cold. For
both of these reasons, if a racket is taken directly from a cold car trunk and
immediately put into hard use, the string can shatter.
The solution to
this problem is simple. You can protect the racket from the cold by transporting
it in the heated cab of the car, and/or by carrying it in an insulated racket
bag. If the racket does get cold, give it several minutes to warm up before
putting it into play.
Even with proper care, some string breakage is
inevitable. As a string ages, it gradually becomes abraded and loses resiliency.
Your best bet is to restring before such degradation occurs: newer strings
perform better, and you'll be surprised less often in the middle of a
match.