When filling over redstone components, the updates of where they would power are sent. This is evident with powered rails, repeaters and redstone dust.

How to reproduce:
set /carpet fillupdates false
place down some repeaters or redstone dust, as well as some buds where they would power
or place down powered rails next to a bud
use /fill on the repeaters, redstone dust, or powered rail
The buds activate

This was reproduced in version 1.4.12