An intriguing filler-induced softening effect was observed in filled polydimethylsiloxanes that aged at high temperatures. It was found that the Young’s modulus and hardness of the aged filled elastomers first decreased and then increased with filler fraction, remarkably different from those of the fresh samples which always increased monotonically. This softening effect cannot be explained by the original phase structure of rigid particles embedded in a soft matrix. However, due to the notable chain scissions and oxidative crosslinks detected in the aged neat resin, the softening was attributed to a heterogeneous oxidative hardening during the thermal ageing. It is proposed that filler particles behaved as secondary antioxidants to block the local reactions of free-radical crosslinks, effectively producing a soft elastomeric layer around each particle and thus this softening effect. This proposal is consistent with the fact that alpha-iron oxide, a strong free-radical scavenger, demonstrated the most pronounced softening effect. The proposal was further examined by comparing the mechanical strength at the filler/elastomer interfaces extracted from the wear rate data among the various fillers studied here. It was found that alpha-iron oxide/elastomer interfaces possess the lowest interfacial strength, in good agreement with the proposed mechanism.