Spark Plug External Corrosion

Went to start up my bike for first time this year and noticed there’s a bunch of corrosion on the inner spark plugs, with none on the outer and some on the middle, pic for context, same on both sides, anyone know what might have caused it to corrode in this way? They’re all from same batch, fitted at same time and only in summer last year. Not a big deal to even replace, but want to be careful to not pour a load of rust into the cylinders. 
 

My 79 A1 has 'drain' holes that bleed to back of block from spark plug annulus. My bet is that passage is clogged, water got into the cavity and sat until it evaporated. Use compressed air before removing spark plugs to blow away any loose corrosion. Or, get into the area with a small brass brush, break the scaling up then hit it with compressed air.

My .02, but realize I'm not a kz1300 eggs-spurt... 

Oh, then clear the passage ways.. 
 

In this photo you can see the drain hole next to the spark plug thread.


They are located on cylinders 2-3-4-5.


Here you have one of the outlet holes, below the intake manifold and above the head gasket.

Thanks both, looks like you’ve called it exactly, those channels certainly don’t look clear, will see what I can do to sort them and clear the corrosion.

Awesome!