We consider the effects of chain aggregation during pull out on the adhesion of elastomer-elastomer surfaces fortified with interfacial connector chains. For the case of monodisperse end-grafted A homopolymer chains at an A/B elastomer interface, we show that lateral aggregation of connector chains in the gap into tethered micelle-like structures occurs at a characteristic length zeta similar to 1/sigma set by the grafting density sigma. This is a first-order transition, characterized by a metastable single-fibril state separated from a state of arbitrarily large aggregates by a gap-size dependent energy barrier. As a result, the threshold fracture toughness G(0) as a function of sigma is predicted to have a plateau reflecting a regime of spontaneous connector chain extraction induced by aggregation. Experimental access to this plateau depends on the rate of A/B elastomer separation, due to the slow kinetics of the aggregation instability. For the case of polydisperse connector brushes, aggregation processes are predicted to occur on many length scales, leading to a possible enhancement of adhesion promotion compared with monodisperse brushes. We briefly highlight this effect with several examples of polydisperse connector brushes and pseudobrushes.