The correlation between rapid initiation and rapid decomposition in olefin metathesis is probed for a series of fast-initiating, phosphine-free Ru catalysts: the Hoveyda catalyst HII, RuCl₂(L)(═CHC₆H₄-o-OⁱPr); the Grela catalyst nG (a derivative of HII with a nitro group para to OⁱPr); the Piers catalyst PII, [RuCl₂(L)(═CHPCy₃)]OTf; the third-generation Grubbs catalyst GIII, RuCl₂(L)(py)₂(═CHPh); and dianiline catalyst DA, RuCl₂(L)(o-dianiline)(═CHPh), in all of which L = H₂IMes = N,N′-bis(mesityl)imidazolin-2-ylidene. Prior studies of ethylene metathesis have established that various Ru metathesis catalysts can decompose by β-elimination of propene from the metallacyclobutane intermediate RuCl₂(H₂IMes)(κ²-C₃H₆), Ru-2. The present work demonstrates that in metathesis of terminal olefins, β-elimination yields only ca. 25–40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy is attributed to competing decomposition via bimolecular coupling of methylidene intermediate RuCl₂(H₂IMes)(═CH₂), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl₂(H₂IMes)L_n(═CH₂) (L_n = py_n′; n′ = 1, 2, or o-dianiline). These adducts were synthesized by treating in situ-generated metallacyclobutane Ru-2 with pyridine or o-dianiline, and were isolated by precipitating at low temperature (−116 or −78 °C, respectively). On warming, both undergo methylidene coupling, liberating ethylene and forming RuCl₂(H₂IMes)L_n. A mechanism is proposed based on kinetic studies and molecular-level computational analysis. Bimolecular coupling emerges as an important contributor to the instability of Ru-1, and a potentially major pathway for decomposition of fast-initiating, phosphine-free metathesis catalysts.