How do accretion disks form?
Basically, that is the result of the gravitational attraction of the masses.
"Accretion" simply means the grouping or pulling together of something. ALL mass exerts proportional gravitation on other masses. Sufficient loose mass eventually "collapses" or accretes into larger collective masses.
An accretion "disk" may form before a planet or moon forms from it, or it may remain as a relatively stable collection of smaller masses.
A LARGE amount of mass may eventually accrete to form a Star.