Are the dental hygeine gods against me?

A funny thing happened this weekend: My dh works for my Dad’s dental office. About a year ago, they went to a dental convention, here in Seattle. At those things, they are frequently give samples of various products in hopes that they will buy these things for their office. Among the products that can be sampled are toothbrushes. Dh has a great time collecting these things and sometimes brings them home to try out before they buy them for the office. My toothbrush was ready to be replaced and when I pulled out my stash my hand landed on some of those from the convention. With the first two, I couldn’t even finish brushing my teeth before the head started to break apart in my mouth! With the third one, I finished, rinsed the brush and took a long look at it. There were fine cracks starting to appear in the head! I continued looking at it every few minutes and in about 15 minutes, this third brush had broken apart as well! I had a similar situation the next morning when I pulled out a fourth brush, but this time I did get through one full brushing of my teeth. Some of these brushes are ones that they have been buying. When I saw my Dad on Sunday afternoon, I told him about this. He said that I have been the only one he knows of to have this kind of problem. My response? “I must have very corrosive saliva!” I now need to go to the store and buy a toothbrush...
ETA a response to the comments: These toothbrushes were stored under the bathroom sink, in their original, plastic packaging. I remembered, after posting this, that I put them there because one of them did this when I tried using it when it was brand new. I did get one full use out of it and then it broke overnight. I had forgotten about that until now.