Over the years running up to 1997, the Thai real estate market had been in a boom. As a result of this, there was a significant amount of investment into the Baht from non-Thai investors who wanted to get in on the boom.
In the mid 90s, there were a series of events that made other countries more enticing relative to Thailand - the Yuan and Yen had both devalued, the US economy was booming (so interest rates there went up), and a sharp fall in semiconductor prices slashed Thailand´s overseas earnings.
As a result of this, more and more people (both Thai nationals and foreigners) wanted to sell Baht to buy dollars / yuan / Yen / whatever.
When more people want to sell a currency (compared to the recent / immediate past), that currency normally goes down a bit in value when measured against the currency those people are buying. The market finds a new exchange rate at which both buyers and sellers are willing to do deals.
The reason this was a collapse rather than a slow decline was that the Thai government wouldn´t allow the exchange rates to change - they had pegged the Baht to the US dollar, and it was only to buy and sell at that rate.
In order to make this happen, the government basically had to use its reserves of foreign currencies to try to manipulate the market (by providing enough buyers, therefore using up Thailand´s dollar reserves) to meet the requirements of the sellers.
The Thai government turned out not to have enough dollars. They were forced to remove the peg.... and the currency immediately crashed as the fact that the exchange rate had been held artificially high for months became apparent.
In fact, when crashes happen, they tend to go too far, then revert, so the Baht doubtless plumetted to an even lower level than it would have reached had the currency floated against the dollar all along.
To complicate matters, once enough big speculators think that a crash is inevitable, they make large transactions that will A: make sure they profit from this crash, and B: (maybe) make the crash even more likely to happen sooner. It´s an open question whether this speculators REALLY cause the crash, or simply mean it happens a bit faster.
Politicians tend to like placing the blame on the speculators, because they give them a target for the ´whose fault was it´ that´s easier for the population to swallow... ´The crash wasn´t about our irresponsible economic policy - it was all the fault of foreigners who wanted us to do badly.´