It is called 'Blossom Drop' and affects all fruiting plants, not just peppers. There is a FAQ here about it (linked below) and many discussions that the search will pull up using that term.

Primarily it is a pollination issue and caused by air temps making the pollen tacky and non-viable.

From the FAQ:

It may be caused by the use of excess nitrogen fertilizers or dry windy conditions, but the most common cause is temperature extremes. Tomatoes, peppers and beans are especially picky about the air temps when it comes time to set fruit. If the night temps fall below 55 or rise above 75 or if the day temps are above 90, the pollen becomes tacky and non-viable. Pollination cannot occur. If the bloom isn't pollinated, the bloom dies and falls off.

See the FAQ for more details.

Dave

Here is a link that might be useful: Blossom Drop FAQ