Popped Spark Plugs

So after passing the 100,000 miles mark, the engine decides to play "I'm gonna die on you now!" One of the spark plugs popped out of it's socket and now a new one doesn't seem to want to go in. Dunno how or why it happened, perhaps you can help me figure it out...here are some pix for you engine experts! Click on pictures for a full sized viewing.

(Above) The last one on the right is obviously new, that is what I bought to replace the old ones.

(Above) Another angle for your analytical pleasure.

(Above) Don't know if this view helps with the diagnostic...