Why didn't electroweak symmetry breaking happen earlier than it did? I've read that electroweak symmetry breaking occurred "at a picosecond or so" after the Big Bang (in a source I found).
I can't help wondering why it took so long to get started.  For instance, was something missing up until then, which was needed by the Higgs mechanism?
What feature(s) of the Higgs mechanism, or perhaps what other theoretical consideration(s), was/were seemingly responsible for this very brief delay?
 A: It is the energy scale of interactions  that controls whether the three forces are unified and when symmetry has been broken. The temperature of the universe as it expands gives also the average kinetic energy of the elementary particles that are contained in the universe at that time. That is the energy with which the particles will scatter and interact and produce new particles .
Here is the Big Bang time line:

The unification scale is controlled by the coupling constants: energy behavior

Plot shown is with assumed supersymmetry, without that assumption the meeting point is not clean.
Here is a simplified diagram that shows how symmetry can be broken given the shape of the potential.


Spontaneous symmetry breaking simplified: - At high energy levels (left) the ball settles in the center, and the result is symmetrical. At lower energy levels (right), the overall "rules" remain symmetrical, but the "Mexican hat" potential comes into effect: "local" symmetry is inevitably broken since eventually the ball must roll one way (at random) and not another.

The effective potentials for the interactions depend on the coupling constants.
The strong force has a different energy behavior and separates first, as the  temperature falls and the available energy of interactions decreases, which gives a window for a quark gluon plasma phase. Then with the expansion the interaction energy available falls to the point where electroweak symmetry breaking happens.
So the timeline is fixed by measurements of the coupling constants in the lab and the use of theoretical calculations that describe their energy dependence. 
A: The electroweak transition is a phase transition. I was going to say it's a transition just like melting or boiling, but those are first order transitions and I'm not sure if the EW transition is well enough understood to say what order it is.
Anyhow, melting or boiling happens at a particular temperature determined by the intermolecular forces in whatever is melting or boiling. Likewise the EW transition happened at a specific temperature determined by the weak interaction. The universe started out very hot immediately after the Big Bang and cooled as it expanded. THe EW transition happened when the universe had cooled to the right temperature. It couldn't have happened earlier because the universe was still too hot.
A: higgs field causing electroweak transition. before higgs field w,z boson photon all are massless. but higgs field gives masses to them and symmetry spontaneously break. because if particles having masses then su(2)u(1) symmetry break. all gauge symmetry break if we give mass to the particles.so transition must have to have below 125.6 gev that is the mass of higgs boson.
peoples are thinking about there may be another background field which causes strong and electroweak seperation.so that field can break the underlying symmetry.
