While the disciples were out getting food (4:8), Jesus just had a chat with the Samaritan woman and even managed to show his true identity to herself.
Now the disciples returned only to see Jesus just finished talking to the Samaritan woman -- and they were surprised (4:27). Remember how the Samaritan woman was an unexpected person for Jesus to talk to (different gender, different culture)? That may be why the disciples were surprised. But even then, the disciples did not even bother asking Jesus questions regarding why Jesus would even talk to a Samaritan woman (4:27). Jesus has done some incredible things like water to wine, and unexpected things are what Jesus does (even today). Jesus doing incredible things are part of the reason why we can be in awe of him.
As for the Samaritan woman herself, she went straight to telling her people about Jesus. She didn't even bother with carrying the water jug back (4:28)! She knew that Jesus is too important for other people to miss. When the Samaritan woman presented Jesus, she told them to "come" and "see" and asked if he "could […] be the Messiah" (4:29). In her presentation, she invited the other Samaritans to experience Jesus for themselves. It's one thing to learn about Jesus from church, but it's another thing to experience Jesus for yourself. And with the curiosity, a bunch of Samaritans flocked to where Jesus is to learn about him.