From a rising water table to a natural spring, there are a number of reasons that water may be intruding through your basement walls. Excess humidity, poor surface water management, poor quality soils and old mortar/concrete around pipes holes can cause cracks to appear outside an ageing basement wall.
If trees are planted too close to the house, their roots may also draw water into the walls. Other issues that may cause leaks including your house is being in the path of water flow, eroded pathways and damaged conduits.