Very often when odds drop it means nothing, I remember a match between Go Soeda vs Brian dabul, odds on Soeda went from +2.00 to 1.25 and I remember one guy asking "isnt it value to bet Soeda at 1.25 because its fixed anyway?"
Dabul won easy 2-0 and it left me wondering what caused such a massive odds collapse.
Yeah, I remember that - there was quite a bit of manipulation around then, after a few high profile fixes. That was actually the biggest "reverse move" of the last decade. The next biggest:
Dolgo-Petzschner Toronto 2010: many 10ks on petzschner to win first set. went on to lose after
Koellerer-Ancic Munich 2010: manipulation iirc. Koellerer later banned for match fixing
Russell-Hartfield Casa 2007: don't remember
Clement-Baghdatis UO 2010: short odds chasing I think, then the heat got Baggy
Falla-Andujar Estoril 2011: retiral
The manipulation trick was for somebody to start a rumour, then buy betfair down from 2.00 to 1.80, lots of others jump on the bandwagon. Then the original guys jump on the other side at 4.0 or more :-) Not seen that in a couple of years now.
However, these are relatively rare, matches with odds moves >40% where the end result went against the money. There are more than 5x as many with >40% where the money was on the winning side, and the moves are a lot bigger. For example, Kravchuk-Lee in Moscow 2006, Kravchuk opened at 5.5, closed at 1.46 :-)