What could lead to gynecomastia?

Many. Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. Although there may be many “causes”, most cases are idiopathic (no specific cause). The use of certain medications, drugs, organ dysfunction (kidney or liver), hormonal imbalances etc. May be causative.
Too many to list. The list includes: puberty, excess estrogen, deficient testosterone, aging, medications (many of them), anabolic steroids, certain muscle building supplements, marijuana, liver disease, kidney disease, hyperthyroidism, and even cancer of the breast, testicles, lung, stomach, liver, kidney, or pituitary gland. If you have gynecomastia, the most common causes are benign, but get checked to be safe.
Male breast swelling. There are many possible factors. It is usually one sided tender breast swelling in a male. Some medications for blood pressure or prostate or antacid pills are associated. Smoking marijuana, excess alcohol intake can be associated. Rarely hormone imbalance or testicular disorder. Can also be seen as normal in some young men at puberty, and is temporary.