The sunlight.. I don't think so. The glue fumes ... it is possible , especially if the glue tube was overlying the tail. If it was in some distance from the part it shouldn't cause a such damage.
I suspect that it was the glue used to join the parts. Too much, combined with a thin plastic will result in softening of he material. I've had that happen when using Tamiya Extra Thin with sheet styrene.