He will tell the Cohen saying, I have seen something like a plague in my house (14:35)

Rashi explains that a person should not say that he has seen something which is definitely a plague in the house, but only ‘like a plague’. The source for Rashi is a Mishna in Negaim chapter 12 (Mishna 5). The commentaries there try to explain the reason for saying ‘like’ instead of definitely. Look at the Tosefot YomTov there (who brings many different explanations).

Perhaps we can explain the reason as follows. If the person would say that it is definitely a plague, he would be deciding the Halacha, that the house is certainly impure. Really this is a task that is given to the Cohen, to decide the status of the house. Sometimes there is a discolouration that look like a plague which are not, and it is the Cohen who is the expert in this. If the house owner decides the Halacha he is transgressing the prohibition of making a Halachic ruling in front of his teacher, and there is a great punishment associated with this (look in Eruvin 63a).

If so, we can better understand what the Rabbis taught in Torat Cohanim. It states there that the Cohen should give the owner of the house words of rebuke, and say, ‘my son, you should know that the plague only comes onto a house because of speaking lashon hara’. Rabbi Shimon ben Elazar says, ‘also because of pride’.

It is not clear why they chose only those two reasons for the plague, when the gemara in Erechin (16a) gives seven different reasons – what is special about these two?

We can explain based on the Talmud there (Erechin 16a). It was taught: Rabbi Elazar ben Parta says, ‘come and see how great is the sin of lashon hara. From where? From the spies (that Moshe sent to Israel). They only spoke badly about sticks and stones (as the Torah tells us in Shelach Lecha – “They spoke bad things about the land”). If the punishment is so great for speaking about inanimate objects, how much more so will a person be punished for speaking lashon hara about another person!

We see from here that there is even a prohibition of speaking lashon hara against sticks and stones. This explains how the Cohen must rebuke the home owner. If he says ‘I have seen a plague in my house’ (meaning definitely) he is in effect saying lashon hara against his house (since he is not deciding the Halacha or saying it for any constructive purpose), which is forbidden even on sticks and stones, and even on a house. Therefore he must say ‘like a plague’ which is not saying something definitely bad about the building (and which is for the purpose of informing the Cohen that he must come to decide the Halacha).

Rabbi Shimon ben Elazar comes to add that the house owner must not make a definite decision about the matter because that is the task of the Cohen. If the house owner makes the Halachic ruling in front of the Cohen that is a sign of excessive pride, which is one of the reasons that the plague came in the first place.
Therefore the midrash didn’t tell the Cohen to list all the other reasons for the plague, because they don’t have a connection to the difference between saying ‘a plague’ or ‘like a plague’.