When a person cries, tear fluid is secreted by the lacrimal gland seen bulging the conjunctiva. This tear passes through numerous ducts into the conjunctival sack, aided by ocular muscle contraction. From there, it reaches the lacrimal sack and through the lacrimal duct, it is drained into the nasal cavity. when there is a sudden discharge of tear, as while crying, the flued is pooled resulting in congestion.
Another interesting fact is that nature has provided with a flap valve at the terminal part of the canal to prevent tear flued from being pushed back by air, while sneezing or bowling.