I understand that, I meant why it increased so rapidly, not why it increased in the first place.
IMO, the reason was incomplete information. At first, people sort of agreed to a base heirloom price, after the sellers realised 500k for a +1 was def. too much and the buyers realised that 200k was definitely too low, everyone was selling and buying any +1 heirloom for 300k. Then the prices started to gently fluctuate.
It lasted quite long and by that time it was really easy to make cheap money with the market. Delusions on the value of items, people needing money very fast, etc.
Then the patch hit and many balance ajustments were made. People already had started to realise that a +1 item maybe wasn't equal to another +1 item, so very logically now the price differences between high and low demand items are extreme (sometimes more than 1 heirloom point in gold), as some good prepatch items became very bad (courser and arab warhorse mainly).
Before some players started playing full 100k+ equipment, there sure was inflation because everybody was either breaking even or earning money, and more and more people were stopping the retirements (thus not feeding the market with heirloom points).
But now that gear crutchers aren't that uncommon, I do think there will be a general decrease in the price of semi-critical heirlooms like weapons, ammo or horses and convenience heirlooms like shields, peasant stuff, head armor and boots. Critical heirlooms like armor and gauntlets however, will not decrease in price.
Everybody wants them at the moment, and I doubt a general decrease in the gold mass will affect those that are able to buy heirloomed armors. Fact is, most armor related trades are direct heirloom point trades, and many people don't even sell their heirloom points despite the somewhat exaggerated price (400k+ while you can find nearly all the +1 heirlooms you want at 300k), they just turn them into heirloomed armor. Very few people have armors to sell, because they are sold very fast to players that actually use them.