Droppers activate when either they or the block next to them get powered. When two droppers are next to each other and one gets powered, the powered dropper activates first. As a result, if a dropper (A) holds and item and points into another dropper (B), then a single redstone pulse directly powering A will cause the item to be pushed from A into B and then out of B. However, this does not happen if there is water next to B and the redstone pulse into A comes from an observer.

Steps to reproduce

1. Place two droppers, with one (A) pointing into the other (B).
2. Place an observer pointing into A.
3. Put a item in A.
4. Update a block in front of the observer.
5. Place water next to B. (This can be source water, flowing water, or a bubble column.)
6. Repeat steps (1) - (4).

Expected result

In step 4 the item ejects from B.

Observed result

In step 4 the item ejects from B when no water is present, but does not eject from B when water is next to B.
Water slows dropper.mp4