Formation and decay mechanisms of photoinduced nonlinear vibronic excitations (``breathers'') in several conjugated polymers are studied using a quantum-chemical excited state molecular dynamics approach. We identify specific coupled vibrational modes responsible for breather excitations and investigate their dependence on chain length. In addition to intermolecular relaxation mechanisms, our calculations show that intramolecular vibrational energy equilibration results in a decay of breathers on a timescale of hundreds of femtoseconds.