that is reason why是由why引出来润饰reason 的定语从句。中辞意思是“那是……的原因”
比方，他爷爷抱病住院他不得不去照顾他爷爷，那就是他为什么这段时光常常迟到的原因。
For example, his grandfather is ill in hospital and he has to take care of his grandfather, and that is the reason why he is often late during this time.