Air carrier pilot reported during parking at their gate the aircraft rolled backwards due to being on a steep incline.

Narrative

During taxi in to gate XX at ZZZ; we had discussed as a crew that there was a note for our gate that said 2 engine taxi in recommended" due to steep incline. I believed that wouldn't really apply to us; since we were in a Max and were fairly light. I thought the plane would have enough power to make it into the gate on one engine without any issue. Nevertheless; we decided to keep both engines running until we were on the line very close to the chocks. As we got close to parking; I called for engine 2 shutdown; and almost as soon as we shut it down I felt us slow down tremendously. As I started adding power to about 40-45% N2; I felt us begin to roll backwards. We set the brakes and realized we needed to start the 2nd engine back up. In the push to get the second engine started; we forgot to configure the bleeds for engine start; and when we finally did the EGT gauge started flashing. No limitations were exceeded; but we aborted the engine start; unsure of what was going on. We then called operations to ask them to tow us to the gate; since I was a bit frazzled at this point and that felt safer. After we got the tow-in coordinated; I instinctively set the brakes and turned off the seatbelt sign. I didn't even realize it; but I turned it back on as soon as the First Officer pointed it out; and immediately told the flight attendants not to open the doors yet. I then made a passenger announcement asking everyone to remain seated. A retired pilot came to the flight deck after the flight to inform me that I had made the announcement too late; and everyone had gotten up and opened the overhead bins."