I've found out today that Unity's ScrollRect is behaving erratic when more than one touch is used for drag. I think that's a bug with UI. It jumps between fingers (to moving one), this script fixes it by blocking event data based on touch id.

Scripts/Utilities/ScrollRectMultiTouchFix.cs

@@ -0,0 +1,45 @@
+/// Credit Erdener Gonenc - @PixelEnvision
+/*USAGE: Simply use that instead of the regular ScrollRect */
+
+using System;
+using UnityEngine.EventSystems;
+
+namespace UnityEngine.UI.Extensions
+{
+	[AddComponentMenu ("UI/Extensions/ScrollRectMultiTouchFix")]
+	public class ScrollRectMultiTouchFix : ScrollRect
+	{
+
+		private int pid = -100;
+
+		/// <summary>
+		/// Begin drag event
+		/// </summary>
+		public override void OnBeginDrag (UnityEngine.EventSystems.PointerEventData eventData)
+		{
+			pid = eventData.pointerId;
+			base.OnBeginDrag (eventData);
+		}
+
+		/// <summary>
+		/// Drag event
+		/// </summary>
+		public override void OnDrag (UnityEngine.EventSystems.PointerEventData eventData)
+		{
+			if (pid == eventData.pointerId)
+				base.OnDrag (eventData);
+		}
+
+		/// <summary>
+		/// End drag event
+		/// </summary>
+		public override void OnEndDrag (UnityEngine.EventSystems.PointerEventData eventData)
+		{
+			if (pid == eventData.pointerId) {
+				pid = -100;
+				base.OnEndDrag (eventData);
+			}
+		}
+
+	}
+}