I would try to present an emotional approach to explain this situation.
First of all, I guess it's sensible to describe disciples' feelings about their life and their Master. Let me try:
1. Dissapointment - their faith in Jesus was touched by events of His death, taking into account: hope for continous friendship with Master, elevating words of Jesus that made them 'Disciples' instead of being just simple fishermen, and so on.
2. Loss of faith in 'better world' driven by Jesus' good deeds and His words of justice, brotherhood and acceptance.
3. Their social position (as disciples) was cut because of not fulfilled wishes of many people of Israel (including being freed from Roman occupation).
4. Feeling of being lost and confused inside their own lives (they put on a stake lots of things, including what we would call 'careers').
That said, I understand their behaviour (being afraid) as:
1. Being mentally separated from other people (because of being ashamed of lost of their 'cause') made them being afraid of people.
2. As Jesus' disciples they may be afraid of someone that would deliver them to soldiers to punish them for taking part in 'rebellion' (as occupants may see it).
3. If they recognized Jesus, they may be afraid of His words about not helping Him against Orthodox Jews and Romans (I know that from our point of view it may be nonsense, but still, these are emotions, not a logical approach).
4. There are some psychological terms that describe our feelings as defence mechanisms, some may apply here.
Last, but not least, we may not be able to fully understand how really felt disciples (translation problems).
I guess that we can try to search for similiar feelings in our inner life. I believe that taking disciples' emotional life may bring some light here.
(I'm sorry if my english in not sufficient).