Why user mostly drops from shipping address page in checkout?

We have a 4 step checkout process in our ecommerce website.

  1. Login/Signup
  2. Shipping Address
  3. Delivery date and special instructions
  4. Payment

Here we have a checkout conversion of around 40%. Means if 100 people signup on our website only 40 will place order. Now we were analyzing behaviour of rest of 60% people who drop from checkout. And we found out that out of these 60 people who dropped, 50% people did not filled their shipping address, they simply signup and leave checkout on shipping address page.

We are not able to understand, why such a big number of people are leaving without doing any activity in checkout. What could be the possible areas where we can improve upon to reduce this number?