Why is one of my friends keeping secrets from me?

So it all started about five weeks ago when me and two of my friends were sitting at lunch in school. We were all talking about stuff when friend B mentioned to friend A that he had to tell him something later. I didn't think to much of it until it happened again and again. Then one day friend B said he would tell friend A something once I was gone. Feeling kinda awkward I just left and sat with other friends. Later I decided to bring up the topic with friend A. He agreed with me that friend B was kind of acting weird. He even mentioned that the secrets weren't that big. I took his word for it because we had been friends ever since second grade. There's only a few questions I have. 1) why is friend B doing this? 2) should I ask friend B why he is acting like this? Just some quick extra details. Me and friend A have been friends since 2nd grade and me and friend B have been friends since 7th grade. We are all guys in 11th grade. Any help would be appreciated thanks