Phimosis can be caused by several things, depending on if it's natural or develops later in life.

Common causes are:

Normal development: In babies and young boys, the foreskin is naturally tight and usually loosens as they grow older.
Scarring: If the foreskin gets injured, it can heal with scar tissue that makes it less stretchy.
Infections of the foreskin: Infections of the foreskin are often caused by bacteria or fungi and can lead to redness, swelling and pain
Poor hygiene: Not cleaning properly under the foreskin can lead to infections and irritation.
Forcing the foreskin back too early: Trying to pull back a tight foreskin too soon can cause small tears, leading to scarring and making the problem worse.

Is it phimosis or agglutination?

Phimosis is different from agglutination. In agglutination, some parts of the foreskin stay naturally stuck to the head of the penis. This helps protect a baby’s penis from contact with urine and poop, which could cause irritation. Agglutination usually goes away on its own as the child grows.