I’ve been practicing idling since last wed, 1st time idling.
Today I felt something funny on pedals, I looked down, and they were about to drop off the pair of them. Why has it done this ?
And is there any way to prevent this from happening again ?
The seat may be the wrong way round. It is important that the left pedal is on the left and the right pedal on the right, otherwise they will unscrew themselves as you pedal. Usually there is a little ‘L’ and ‘R’ somewhere in the pedal or pedal crank. Though on second thoughts, if you have mainly been idling, it may just be that you have been pedalling backwards as much as forwards.
Check the pedals as SW said, but this can happen when you practice a lot of idling or backwards riding. The easiest fix is to just ride forward a few dozen feet every so often while you practice. You could also use blue loctite, but sparingly on pedal threads.
Pedals
Just checked uni, left was on right. Turned seat round. So left now on left etc. See if that does the trick. Cheers !!! 
good luck riding that thing backwards 
er… I mean frontwards
Also, make sure your pedals are installed with a healthy amount of grease on the threads.