The term ‘sardine run’ is part of the cultural heritage of the South African nation and refers to a natural phenomenon that is well known to the general public but still poorly understood from an ecological perspective. This lack of understanding has stimulated numerous hypotheses, often contradictory, that try to explain why (ultimate factors) and how (proximate factors) the run occurs.
Source : African Journal of Marine Science, Volume 32, 2010 - Issue 2 (paywalled)

The 'Run' (a.k.a. the ‘KwaZulu-Natal Sardine Run') usually happens between May and July close to the South African coast. It can be 7 km long, 1.5 km wide and 30 meters deep - involving billions of sardines (more accurately South African Pilchards).

The reasons for the mass migration are unclear. Theories (sometimes contradictory) include • For breeding purpose • To avoid predation • Food scarcity/abundance etc etc.

It has also been suggested that it could be a 'relic' (inherited) behaviour, which no longer has any distinct purpose.

Note: The 'return' migration, which follows, is also unexplained.