Eye styes can appear suddenly due to various factors. One common cause is a bacterial infection, often by Staphylococcus aureus, which can occur when bacteria enter the oil glands near the eyelashes or inside the eyelid. Other triggers include blocked oil glands, poor eyelid hygiene, using contaminated makeup or contact lenses, or certain underlying health conditions like blepharitis. Changes in hormone levels, stress, or a weakened immune system may increase the likelihood of stye formation.