Pop-up Ads Craze in the Blogosphere... Why? How?

Yesterday, I was bloghopping and I read from Avee's post that she is experiencing pop-up ads. I didn't give much attention to it because I don't know why it happened and I thought it is only an isolated case. Today, sis Avee left me a message in my CBOX in my Sweet Lullaby blog informing me that I also have pop-up ads. Honestly, I really don't notice it because my pop-up blocker is on. So, I tried to turn it off and there I notice that there are really pop-up ads that appeared twice so far tonight. Grrrrrr..... Oh, how I hate it... It is annoying! In the first place, I don't remember that I have a contract with someone or any company for them to have this legally attached to my site. Both my blogs are on my own domain although it is freely hosted at blogger. I managed my sites and I am the only person who has access to the settings and stuff except of course if there is someone who hacked this. I really don't know... I am confused... I am just making an assumption here... I know I am not alone in this situation right now. Ladies and gentlemen, please inform me with any update that you will know on this "pop-up ad mania" that is going on in the blogosphere right now. Please... I am curious to know how and why this happens...