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We examine the stellar [$\alpha$/Fe]-[Fe/H] distribution of $\simeq1000$present-day galaxies in a high-resolution EAGLE simulation. A slight majorityof galaxies exhibit the canonical distribution, characterised by a sequence oflow-metallicity stars with high [$\alpha$/Fe] that transitions at a "knee" to asequence of declining [$\alpha$/Fe] with increasing metallicity. Thispopulation yields a knee metallicity - galaxy-mass relation similar to thatobserved in Local Group galaxies, both in slope and scatter. However, manysimulated galaxies lack a knee or exhibit more complicated distributions. Kneesare found only in galaxies with star formation histories (SFHs) featuring asustained decline from an early peak ($t\simeq7~{\rm Gyr}$), which enablesenrichment by Type Ia supernovae to dominate that due to Type II supernovae (SNII), reducing [$\alpha$/Fe] in the interstellar gas. The simulation thusindicates that, contrary to the common interpretation implied by analyticgalactic chemical evolution (GCE) models, knee formation is not a consequenceof the onset of enrichment by SN Ia. We use the SFH of a simulated galaxyexhibiting a knee as input to the VICE GCE model, finding it yields an$\alpha$-rich plateau enriched only by SN II, but the plateau comprises littlestellar mass and the galaxy forms few metal-poor ([Fe/H]$\lesssim - 1$) stars.This follows from the short, constant gas consumption timescale typicallyassumed by GCEs, which implies the presence of a readily-enriched, low-mass gasreservoir. When an initially longer, evolving consumption timescale is adopted,VICE reproduces the simulated galaxy's track through the [$\alpha$/Fe]-[Fe/H]plane and its metallicity distribution function.

Realistic simulated galaxies form [$\alpha$/Fe]-[Fe/H] knees due to a sustained decline in their star formation rates