It is possible to construct latex documents that take any number of runs to converge, or in fact which never converge at all.
The most obvious reasons for changing after the first run are references and citations being resolved so changing ?? to 1.2.3 so changing the text length, and the table of contents and list of figures being added after the first run.
However the table of contents added after on the second run is based on page numbers collected in the first run, so after all the references have been resolved, the text has different lengths so page breaks can change and the table of contents can be wrong and require another latex pass. Any change at all in a line break in one pass might just tip a decision to place a float on one page rather than another, so affecting the list of tables and potentially the table of contents.
If your front matter uses (say) a roman numbering and the main matter uses (say) arabic and there are not too many page references, most documents settle down after two or three runs however it is possible to create pathological documents where each time a reference is resolved it affects the page breaking and invalidates all later page references until the next run, usually page references to pages numbered with roman numerals are involved (as in the answer linked above) as the roman numbering scheme has the useful property that the width of the printed number can decrease as the number increases, eg
v which combined with long enough numbers that they need to hyphenate can lead to interesting interactions between the page breaker and the