What causes a basement to flood?

When a property with a basement is constructed, a hole in the ground must be excavated, in which the basement is formed. Once complete, soil is backfilled around the building.

Digging the soil up and then backfilling it breaks it up, leaving it loose in comparison to the denser virgin soil which remained undisturbed around it.

The result is a 'bowl' of denser soil with a property within it surrounded by the loose backfilled soil.
Rainfall can then percolate in to the looser backfill, and build up between the walls and denser soil, asserting hydrostatic pressure against the walls.
A Common feature is the inclusion of a land drain, which collects water at the point where it builds up at the bottom of the 'bowl', causing it to drain safely away before pressuring into the basement. The limitation of this is technique is that this system sits within the soil and can therefore clog over time as silt is washed through the drainage pipe. Many land drains are installed without provision of means for cleaning them out, so that eventually, once dry basements become susceptable to flooding.
The photo's opposite illustrate silting with external drainage systems.
When drainage inevitably breaks down, hydrostatic ground water pressure comes to bear against the walls and floor, and can then penetrate, most commonly at the wall and floor joints, causing your basement to become damp or flooded..