There is a venous plexus, or cluster of veins, under the eyelids.
Anything that irritates the eyes or the mucous membranes in the lower eyelids can cause those veins to get boggy and accentuate them. Allergies
of course can, but so can illness, and simple fatigue. Sleeplessness causes both eye strain and eye irritation
, and rubbing can further irritation, causing more bogginess.