Nest box mystery (1 Viewer)

We've enjoyed a camera-box for about five years now. It has been occupied every year and still gives us the best programmes on the TV.
When purchased it had three aluminium plates so we could choose the target species and we opted for Blue Tit as the most appealing of the common garden birds.
In year three, at the beginning of the season, I could clearly hear a pecking sound in the box, even though I was indoors and on the TV I could just see frantic activity in the entrance hole. This was repeated in year four but not this year - probably a new bird.
OK, getting to the point, I took off the front panel and you can see how much wood has been removed and this is the mystery to me. Enlarging the entrance hole would serve no useful purpose and would make it easier for predators to get in. Of course, the aluminium plate prevented this but why should the bird try to do this?
This year, no chipping, first egg on 12 April and this morning she's brooding and being fed by an attentive male.

my first thought was it is not uncommon for great spotted woodpeckers to attack nest boxes in this way to predate on the eggs/fledgelings. however, from the picture the damage appears to be from the inside so it does indeed seem a strange behaviour

Yes, Paul, it's been done by a Blue Tit from the inside - dreaming of growing up into a GSW perhaps? Good thing it's not a concrete next box. Interesting that this seems to be very uncommon.