I've noticed this a lot recently on single carriageway roads.
When I've seen an overtaking opportunity I pull a bit closer to (but not tailgating) the car in front ready to pull out to overtake.
Just as I'm about to pull out, the car brakes, immediately cutting the distance between me and it, meaning that I either have to commit to the pass a split second earlier than I intend, or abandon it.
Its as if they panic and slam on or they think they are helping. Either way, I wish they wouldn't do it.