Google AdWords Quality Score may be giving more critical evaluation to campaigns which are consolidated to one domain.
If that domain has had issues in the past with Google results, unethical SEO, cloaking, or other techniques which may cast it as being a negative in the eyes of Google, hosting the landing pages on such a domain may bring down its Quality Score.
From a Digital Point thread:
Clearly there is a domain name Quality Score that is affecting us. It seems when using individual domains names the success or failure of an individual ad group/keyword does not affect the others, but when it all goes to the same domain name it seems tied into each other.
Of course, if your domain is clean and Google considers it an authority, hosting AdWords landing pages on that domain can be beneficial.