I didn’t understand ____ this would happen.答案给的是why，为什么我填how

这道题应该是在一定的语境下，应该是在这篇完形填空里的吧：
Last year, my brother and I went to Miami for a vacation. Some of my friends who had been there before said_16_____ was a wonderful holiday destination. Before we went, we had planned for months. When the day came, we were ready.
After our plane landed, we went to the hotel. We had made our reservation six months__17____（early）, but the man at the front desk said there had been a mistake. We 18_____（tell）that our rooms hadn’t been reserved for that week, 19_____ for the week after. I didn’t understand 20____ this would happen. What’s worse, the hotel had been fully booked. When we were wondering what to do, the manager came out. She was 22_____（surprise）helpful. She apologized for the mistake and gave us a spare VIP room on 23_____ top floor. We had never stayed in such an amazing room, and we weren’t charged extra.
The next day, my brother and I went to the beach 24____ we watched some people play volleyball. We got a little_____（sunburn），but the day had been so relaxing that we didn’t mind.
【解析】20、I didn’t understand why this would happen
我不明白为什么会发生这样的事情，本句是一个宾语从句，而且为了这次预定我的信用卡已经被收费了。名词性从句的连词的选择主要通过上下文的语义实现。根据句意说明使用why。

why表示为什么会发生
how表示是怎么发生的
一般都会发表感慨，为什么会发生这件事呢？