Whenever I buy some common fish eggs when the tiny fish appear some of my common fish dissapear and suddenly appear with the tiny newborn fish. What is happening?

Your fish aren't disappearing, they are "relocating!"

You're seeing the results of some of the fish in your tanks rapidly traveling to the upper left corner of the tank at the exact moment that the new fish hatch. Fish will also "warp" to different places in the tank at various times. This is a phenomenon that has been in every version of Fish Tycoon since the Palm days, and it's a harmless (if annoying) function of how the tank updates are performed.

I see this happening a lot, esp. with either eggs or pregnant fish just after they "give birth". Basically they disappear for a second and then reappear at the top left of the tank.

I figure it has something to do with programing and adding baby fish. No biggie.

i guess so but sometimes it makes me think that my fishies are rapidly growing!