There can be multiple reasons for it, but as your husband has recently had the bypass, it probably is caused by the heart not pumping very well at the moment.
If all is well, the triple bypass will make the blood supply to the heart much better and enable the heart to recover from the lack of oxygen it suffered from .
A bypass surgery is a major surgery and your husband will need time to recover from it. Try to be patient, it still is very early. It may take months for him to fully recover. During these months, he will slowly get better and hopefully at some point, not need the oxygen anymore.
Take care