The cross itself, of course, is a symbol of the Christian faith. Anything seen as gigantic implies that the object has been made into a very big deal, given highly exaggerated importance. [Note this indication would be somewhat moderated if in fact this particular cross was designed to be gigantic as a landmark for example or perhaps hung in a church etc.] Yet by "falling", it is also indicated that there is a failing in the exaggerated importance of the cross. (as it is a gigantic cross that falls here) [Many Christians put all their faith upon the blood and sacrifice of Jesus on the Cross. Could there be more required?]

Falling on someone's head could be equated with being hit over the head. By falling on your husband's head from behind, it might be implied that in the past (behind) the significance of the cross (because of its huge size) was overly impressed upon your husband's mind.

This is something that he can see when facing a glass door. A glass door represents the idea of an option or opportunity that is Truth. Thus facing such a door can simply imply an opportunity to face the Truth.

The Cross here might also relate to the idea of "having a cross to bear" - having some severe hardship to endure. But here again this is a very exaggerated idea that may have been impressed upon your husband in the past and there is a failing in this idea.