What conditions cause glaciers to grow larger and advance?

Gravity and/or a downhill slope causes the glacier to advance. As it slides down, the glacier gathers more material such as snow and dirt, which cause the glacier to grow larger. 

Sequoia national park  /  inyo national forest, california, u.s. mount whitney is the tallest mountain in california, as well as the highest summit in the contiguous united states and thesierra nevada—with an elevation of 14,505 feet (4,421 m).