The culprit was a Flambeau apprentice, who had been tasked with destroying the London bridge for his gautlet, set by an old nostalgic pater who had destroyed it as part of his own gauntlet in 1014, while in the retinue of Olaf II of Norway. In 1091 the bridge had been warded strongly against Ignem; the apprentice's mater wanted to avoid widespread destruction and see if the apprentice could find a clever way around the issue. The apprentice interpreted the notion of "clever" with "steal the covenant's reserves of Auram vis and spont-up a severe weather phenomenon"...
The apprentice did, however pass the gauntlet, and remembered the whole episode with sufficient fondness that destruction of the London bridge was part of the gauntlet he set to his own filius, 44 years later, in 1135. The Tribunal was not happy at all with the result (the bridge went down once again in flames, though they were the result of meddling with mundanes rather than direct magical action), and fined stiffly the culprit.
Nonetheless, the "destroy the London bridge as part of your gauntlet" had become something of a tradition for the lineage by then (as no less than three generations of magi had done it), so at the time he took his first apprentice in 1160, the Flambeau magus made sure that the bridge was rebuilt (you can't destroy what's not there, right?) under the supervision of Peter of Colechurch. The bridge was rebuilt by 1163, and in 1176 the Flambeau's filia, as part of her gauntlet, manipulated Henry II into having the wood bridge torn down so it could be rebuilt in stone - a clever and remarkably indirect way to destroy the bridge. The Quaesitores disliked the interference with mundanes; but reckoned that the outcome of all previous gauntlets of the lineage had been immensely worse, so they just gave the new maga and her pater a stern admonishment.
Rebuilding the London bridge in stone turned out to be a much more expensive undertaking than it had been expected, and in 1197, when the Flambeau maga took her own first apprentice, it seemed the construction would require several more decades. Unwilling to break with tradition, the maga magically produced a vast quantity of silver, which she subtly directed over the next few years to the coffers of the English king. The resulting inflation forced the Tribunal to make, in 1208, its famous ruling limiting the quantity of magically created silver that magi could put in circulation; but by the next year, 1209, the London bridge had finally been rebuilt. And just in time, because the maga's apprentice would face her own gauntlet in 1212...