Anyone got a chemist friend who could comment on what "vigorously" might mean in this event?
Halons are haloalkanes -- carbon skeletons with fluorine, chlorine or bromine instead of hydrogen atoms.
The most common ones used as suppressants are called Halon 1211 and Halon 1301 (respectively, CF2ClBr and CF3Br).
The FAA did some tests on Halon 1301 and Li-Ion batteries a decade ago:
http://www.fire.tc.faa.gov/pdf/04-26.pdf
From the executive summary:
"Halon 1301, the fire suppression agent installed in transport category aircraft, is ineffective in suppressing or extinguishing a primary lithium battery fire. Halon 1301 appears to chemically interact with the burning lithium and electrolyte, causing a color change in the molten lithium sparks, turning them a deep red instead of the normal white. This chemical interaction has no effect on battery fire duration or intensity.
The air temperature in a cargo compartment that has had a fire suppressed by Halon 1301 can still be above the autoignition temperature of lithium. Because of this, batteries that were not involved in the initial fire can still ignite and propagate.
The ignition of a primary lithium battery releases burning electrolyte and a molten lithium spray. The cargo liner material may be vulnerable to perforation by molten lithium, depending on its thickness. This can allow the Halon 1301 fire suppressant agent to leak out of the compartment, reducing the concentration within the cargo compartment and the effectiveness of the agent. Holes in the cargo liner may also allow flames to spread outside the compartment."
To answer the question, it does not appear that the vigorousness of the reaction with the halon is the main thing; rather, it is the participation of the intended suppressant in the fire and its consequent failure to extinguish it. The report suggests that the halon did not make the fire worse (which you would expect if the reaction added anything to the combustion process).
From p 9-10 you can see that the Li reaction is indifferent to the presence of the halon, but other reactions are affected:
"The color change of the lithium sparks indicated that a reaction was occurring between the lithium and the Halon 1301. This reaction had no effect on the fire progression, neither hindering nor promoting the spread of the battery fire. The vented electrolyte fires, normally pale red in color, turned bright red when exposed to Halon 1301.
The battery fire continued to propagate until all batteries were consumed, continuing long after the 1-propanol fire was extinguished. The halon also had no effect on the peak temperatures in the test chamber, peaking at about 1400°F. This is similar to the peak temperatures exhibited in previous unsuppressed fires. However, the overall temperature profiles were lower, due to the extinguishment of the 1-propanol and battery plastic coating fires."
The FAA also tested Halon 1211, which is also ineffective:
http://www.fire.tc.faa.gov/pdf/09-55.pdf
I suspect this is likely because lithium is very reactive and, compared to fluorine and chlorine, bromine is the most reactive when attached to a carbon atom (the basis for, e.g., Grignard reagents). The two together are thus able to undergo reaction, especially given the temperatures produced by the primary failure. The whole thing is worth reading in its (scary) entirety.
All in all, a Bad Thing.