Plumbing:
Plumbing equipment usually develops leaks due failures in joints or hose attachment.
Internal wall leaks are even harder to discover and can cause major damage.

Pipes
The common problems in pipes are blocked toilets and drains (especially kitchen drains), failure of garbage disposals, and roots in sewer lines .

Weather:
Unpredictable summer storms, floods, and other water accompanied disasters such as hurricanes, blizzards, and tornados are some more reasons for water damage.

Heating Systems and Air Conditions:
Neglecting air conditioners leads to poor performance, expensive operating costs and moisture issues. Moist cooling ducts promote mold and other water-borne bacteria.

Appliances:
Water heaters, washers, and dryers tend to crack over time, allowing water to trickle. Although these appliances will start leaking slowly, this leak can grow and cause a flood when completely failed. Hoses tend to deteriorate inside out, resulting in leaks as well.

Basements:
A wet basement is caused by moisture accumulations that drip through the foundations. When moisture evaporates, it increases humidity even if there are no leaks or standing water. Another common cause is condensation on the cold concrete walls and floors during more humid time periods.

It is recommended to inspect your home periodically to ensure that it stays in good condition and keeps its real market value. A periodic inspection will also help preventing high-costs restoration projects after water related disasters.

Additional inormation includes:

How to prevent water damages?
What are the visible and invisible causes of water damage?
Best ways to deal with water damage disasters.
How to self inspect your home and be ready for water damage
Water damage insurance issues and services.
How to find and hire a water damage restoration contractor.
And much more