Selling your home is a big decision and I have found in the past that people go from no thoughts of selling to listing their home usually in just a matter of days. Why is such a big decision so impulsive? Well I think there are a few reasons.
1. Money Problems - If there are money stresses in your housgold and you suddenly realize that you can use some equity in your home you may just go ahead and sell.
2. Move up, move down ideas - Empty nesters do not think of themselves as empty nesters until they see it one day and decide to move into a condo. This is ususally a lifestyle type decision that gets sparked by an add or a casual conversation.
3. Friends buying or selling homes - If someone at work is looking at new homes they will talk, this may get you interested as you hear what their priorities are and compare them with you own and suddenly you have a Realtor in your house and are listing it to sell.
Now that you can see some of the motivations you should also know when you should sell your home. Usually there is a drop in the number of listings on the market in December and January and the sales are still happening so by February or March you will see great conditions for a pricing uptick.
Remember that real estate is a very strict supply and demand market. When there are few buyers and lots of sellers prices drop and as there are lots of buyers and nearly no sellers then the prices will increase.