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Considering that tidal forces normally pull bodies apart, I wondered what forces were involved, and in the PDF found this:

"Several deformation modes are active during typical disruption encounters. In the x-y orbital plane of the binary system, the star will experience tidal stretching along the direction connecting the black hole to the WD, and tidal compression perpendicular to this direction. The star will also undergo tidal compression in the direction perpendicular to the orbital plane. Compression perpendicular to the orbital plane is significantly greater than that in the orbital plane, and is the primary mechanism that could, in principle, ignite nuclear reactions in the WD."

So parts of bodies not on the orbital plane don't like their own orbits and try to follow ones that cross thru the main plane, and this effect is great enough to trigger fusion during closest approach (several Schwarzschild radii they say). And a white dwarf could make repeated passes, getting a fusion booster shot each time...

Sounds like the setting for an SF story by Robert Forward. :-)