A number of airports, especially the big ones (including LAX) and others that have low visibility on a regular basis, have what are called SMGCS (pronounced "Smigs") procedures that are put into place when visibility at the airport drops below a certain threshold. While these procedures should not normally prevent IFR aircraft (which all Part 121 operations are) from departing, they can cause delays, as they often involve different taxi routes and spacing of aircraft. This and airfield construction are the two reasons LAX generally experiences delays, which can be monitored, along with any other airport nationwide, here: http://www.fly.faa.gov/flyfaa/usmap.jsp