I find it helpful to consider how forgetting one’s learning is midda-kenegged-midda for leaving an open sefer unattended.
First, I think the term ‘sefer’ in this context originally referred to scrolls, though these days we apply it to books as well.
Scrolls have a tendency to roll off a table, which would be a bizayon. Two main reasons one leaves a scroll in such a precarious position are either one is momentarily distracted by some other task and “forgets” about the sefer or that one is worried about “forgetting” one’s place.
Both reasons show a weakness in one’s appreciation of the kedusha and value of the sefer and it’s contents. A more dedicated attitude towards the materials one studies from also influences the person to retain that learning.