Extended pistons retract for a second when the world is loaded.

From MCPE-19086:
The extended state of pistons is not saved in the file, instead their state is recovered on the chunk load by ticking them with some delay.

In the screenshot shown above, the sticky piston is permanently powered and thus should never retract. But if you reload the world the piston momentarily retracts and triggers the dispenser containing a water bucket.

Original description:
When loading the world extended pistons that aren't in the same chunks has the player will retract for a second. It seems to effect 3 to 4 chunks away from the player.