If it stores too much electricity, its behavior turns aggressive. To avoid this, it occasionally discharges excess energy and calms itself down.
Biology
(from Bulbapedia)
Raichu is a bipedal, rodent-like Pokémon. Raichu is covered in dark orange fur with a white belly. Its bifurcated ears are brown on the outside, yellow on the insides, and end in a distinctive curl. There are circular yellow markings on its cheek where its electric sacs are, and it has a triangular, dark brown nose. Its arms and feet have patches of brown fur at the end, and the soles of its big feet are tan with a circular orange pad. On its back are two horizontal brown stripes. Its long, thin tail has a lightning bolt-shaped end. This lightning bolt is smaller on females. The tail is used to gather electricity from the atmosphere, or it can be planted in the ground to search for electricity. Its tail also serves to protect itself from its own power.
Raichu exudes a weak electrical charge from all its body and slightly glows in the dark, and is capable of storing over 100,000 volts of electricity. However, it will become aggressive if it has stored too much electricity. To keep from reaching this state, it discharges electricity through its tail into the ground. This leads to scorched patches near its nest. If Raichu's sacs are fully charged, its ears will stand straight up and its muscles become stimulated. Raichu is found in forests and woodlands. Like the two others in its evolutionary family, Raichu is able to learn Volt Tackle as a signature move.