Program prices up-down-zero-up...

As I already mentioned in the past I am using daily the AppShopper to hunt for free and reduced prices of programs.
I notice quite a few times the type of "Roller Coast" the prices are going through.
Some times I wander why a program that cost 2 month ago $2 becomes free.
Is it because no one bought it and the producer gave up and threw it away for free , or something else .

here is one example: