What triggers magnetar bursts and giant flares?
In plain words
Magnetars sometimes release up to about $1e46\,\mathrm{erg}$ in a fraction of a second, possibly when their solid crust cracks under magnetic stress or when twisted magnetic field outside the star suddenly reconnects.
Precise statement
Giant flares (SGR 0526-66 in 1979, SGR 1900+14 in 1998, SGR 1806-20 in 2004 with $E \sim 2e46\,\mathrm{erg}$, and the extragalactic GRB 200415A) and short bursts ($1e37 \text{ to } 1e41\,\mathrm{erg}$) require a trigger. Decide between crustal failure (starquakes, plastic flow) and magnetospheric instability or reconnection of twisted fields, using millisecond rise times, quasi-periodic oscillations of $18 \text{ to } 1800\,\mathrm{Hz}$ in flare tails, burst energy distributions, and associated timing changes (glitches and anti-glitches). Answer: the trigger with quantitative evidence.
What would settle it
Simultaneous X-ray timing and spectroscopy of many bursts and a future Galactic giant flare, compared with predicted rise times and oscillation spectra of crust and magnetosphere models.
Status in the literature
2021: a giant flare from a magnetar in NGC 253 (GRB 200415A) was identified, showing such flares appear as extragalactic short gamma-ray bursts.