We present a detailed investigation of the dynamics of laser-excited quasiparticles in YBa2Cu3O7 thin films below the critical temperature. Reflectivity transients at low temperature trace the generation and recombination behavior of quasiparticles. The quasiparticle cascading and recombination rates are determined by comparison with a detailed nonlinear model of the quasiparticle dynamics based on extended Rothwarf-Taylor equations.