I do agree with the stretch reflex, and the fact that when one starts a DB press, elbows tend to be below parrallel (to the floor), the DBs are at a stand still, thus making the weakest part of the lift, the concentric portion, even harder.
I would add that there may be a neurological reason behind this. No matter how psychologically amped or ready you may feel, your neurons may not be. This is where an adequate warmup may be beneficial. For myself, especially on my first lift, I warm up in a particular fashion. If I was doing squat for example, and my goal was 10 reps with 2 plates my warm up would consist of something like this:
95lbs - 10 reps
135 - 8 reps
185 - 6 reps
225 - 1 rep
I find the 1 rep, especially when doing a higher number of reps during my set, to benefit my lifting. My body comes to understand what 225 lbs feels like and gets accustomed to it both on a physioligical and psychological level.
I have a friend who does something quite different. As an example, if he was to squat 225 10 times, he may place 275 on the bar and do a couple of squats with it, then drop the weight down for his set.
To each their own I guess, just as long as it increases your performance.