From AMS Glossary
The extremely low frequency (ELF) component of some sferics that lags the initial very low frequency (VLF) arrival because of the lower phase velocity at low frequencies.
The cause of the slow tail is still controversial. One explanation attributes it to the continuing current. The lightning radiation arrives after the initiating ground stroke because 1) it has components below about 3 kHz in frequencies (necessary for the propagation in the earth–ionosphere waveguide cavity), and 2) it suffers dispersion as a result of the waveguide propagation that broadens the original waveguide impulse, making it look like a tail.
Wait, J. R. 1960. On the theory of the slow-tail portion of atmospheric waveforms. J. Geophys. Res.. 65. 1939.