  The decade-long Vista Variables in the Via Lactea (VVV) survey has detected
numerous highly variable young stellar objects (YSOs). We present a study of 61
highly variable VVV YSOs ($\Delta K_s$ = 1-5 mag), combining near infrared
spectra from Magellan and VLT with VVV and NEOWISE light curves to investigate
physical mechanisms behind eruptive events. Most sources are spectroscopically
confirmed as eruptive variables (typically Class I YSOs) but variable
extinction is also seen. Among them, magnetically controlled accretion,
identified by H{\sc i} recombination emission (usually accompanied by CO
emission), is observed in 46 YSOs. Boundary layer accretion, associated with FU
Ori-like outbursts identified by CO overtone and H$_2$O absorption, is observed
only in longer duration events ($\ge$5 yr total duration). However, even in
long duration events, the magnetically controlled accretion mode predominates,
with amplitudes similar to the boundary layer mode. Shorter (100-700 days)
eruptive events usually have lower amplitudes and these events are generally
either periodic accretors or multiple timescale events, wherein large
photometric changes occur on timescales of weeks and years. We find that the
ratio of amplitudes in $K_s$ and $W2$ can distinguish between variable
accretion and variable extinction. Several YSOs are periodic or quasi-periodic
variables. We identify examples of periodic accretors and extinction-driven
periodicity among them (with periods up to 5 yr) though more data are needed to
classify some cases. The data suggest that dynamic interactions with a
companion may control the accretion rate in a substantial proportion of
eruptive systems, although star-disc interactions should also be considered.
