This also happened in my first playthrough. It's not like the game actively discourages you to do everything else before going to Moonrise Towers. The Mausoleum is really close to the House of Healing, it really looks like Shadowheart would like to visit it because of the millions of clues about Shar worship in the area, and finally Raphael shows up with a quest for you at the entrance. No one specifically says you should infiltrate Moonrise Towers as soon as possible. It is the obvious course of action if you choose to arrive at Act 2 through the Mountain Pass, but it's absolutely not as obvious when choosing the Underdark path.
