when do high energy photons become a paritcle/antiparticle pair?
Your description is somewhat incomplete. In order for pair production to take place the photon has to interact with a nucleus so that momentum conservation will hold. Alternatively if two photons collide (such as just after the big bang) pair production will also be possible.it can become when its energy is above twice the particle mass.
e.g. pair production of electron + positron is possible when the photon has energy above 2*m_e = 1022keV
photon -> e+ e- violates momentum conservation, as mathman told you.why cant the momentum be transferred to to the particle/antiparticle pair? and what is it transferred to when they collide?