It's likely some church group that has a list of all the stores' fax lines and is spamming them with boycott threats. At my main job, 95% of the faxes we receive are barely a step above email spam.
At my other job, the fax machine only seems to ring when someone mistakenly calls the fax number instead of our main phone line. We used to get spam faxes all the time; I'm glad that stopped. We only have one person who even knows how to operate the fax machine.