It was due to a frozen pitch from what I remember sorry and that's what Statcat says as well.
Ian Rush's autobiography also confirms it was due to a frozen pitch
https://books.google.co.uk/books?id=kKs1KucSOO0C&pg=PA186&lpg=PA186&dq=sunderland+liverpool+january+1985+frozen&source=bl&ots=WluAYJfBG3&sig=yF6AoWLVABFc_O-E0rZUafJjFCY&hl=en&sa=X&ved=0ahUKEwiN0pX5iYrVAhVqBsAKHSdKBIkQ6AEIPzAF#v=onepage&q=sunderland liverpool january 1985 frozen&f=false
Definitely a frozen pitch. I was at both games. We were indeed battering them in the first half and I remember we had a great chance for an opening goal - a cross and a header. The striker (can't remember who) did everything right and headed it down - it looked a goal all the way but the ground was so hard it bounced over the top!
Then the rearranged game I think we kicked off, a pass was intercepted and Ian Rush ran up and scored. I think we got beat 3-0?