Word senses rise and fall due to a variety of causes. Previous research has explored how words grow novel senses, but the opposite problem of word sense decline is much less studied. Inspired by recent work on word decline, we investigate the cognitive factors that might explain the historical decline of word senses. We formalize a set of eight psycholinguistic predictors and assess their roles in discriminating declining senses from stable ones over the past two centuries in English. We find that semantic density, change in usage frequency in the semantic neighbourhood, and contextual diversity all predict word sense decline. Our study elucidates the cognitive underpinnings of word sense decline as the lexicon evolves.