With more than 9 columns (36 wide tiles or 72 square tiles with TileRows=4) the subsequent tile columns fall down on a second row.
With TileRows=2 the breaking point would be 19 wide or 37 square tiles.

Ways around this are:
- separating tiles into more groups, if possible. When databinding the DataGroupNameField is used to point to the desired group name as a field in the datasource
- when creating tiles programmatically you can count them and create new groups when needed
- if you do not need any special functionality provided by RadTileList you can consider creating standalone tiles