Probably a bit of both.  Going to a lighter weight is going to make you feel like you can almost manhandle it, and it can be hard to refrain now that you can suddenly do things you weren't able to do with a heavier weight.  It can also change your timing slightly, so it's not surprising that you're feeling it.  I'd say you've successfully identified two main culprits. 

I made the change to 15lb for the last two years.  I still use some 16lb in practice session and occasionally in league.

Several things I noticed :
1. I tend to get around the 15lb very easily (not good, unless the condition calls for more side rotation), so a conscious effort in setup is required to stay behind the ball.
2. Timing has not been a problem or issue as long as my arm swing remains free.  (no muscling). 
3. The further the move inside might favor the 16lb ball.  (speculation on my part)

As far as scoring I haven't noticed any difference (maybe a small favor to the 16lb from deep inside line).
But the jury is still out.  Barring any injury it is nice to have the option and the buying choice to use either weights.
 

I made the change from 16 to 15 years ago and I had timing issues as well. One thing you could also try is to drop the ball a few inches from where you normally have it when you set up. My proshop guy recommended I do that when I told him of my issues and it worked great for me. I didn't have to change my footwork at all. I think it just depends on which one you're more comfortable changing(feet or starting position of the ball).

I would think changing back and forth would create problems too.