A characteristic of certain hot-melt adhesives
which causes them to attempt to return to the
shape or configuration they had when they set.
When a hot-melt adhesive is applied to the flat
spine of a book, and the book is subsequently
rounded and backed, the "memory" of the adhesive
tends to cause it to revert to the original flat
configuration it had when it set, thus also
flattening the spine of the book. This loss of
round will eventually cause the binding to fail,
i.e., the book will become loose in its case
because of the strain of the backing shoulders
caused by the loss of round.