The other guy ive been seeing told my bf everything, why did he do it ?

my boyfriend found out bout the 'other guy' Im seeing last night in a big scene at my apartment.My boyfriend was there waiting for me, when the 'other guy' dropped me off as he was helping move some stuff., then the 'other guy' got very jealous when I asked him to drop me off before my house, as I knew my boyfriend was there waiting for me, well he got pissed and said no; and drove upto my doorway where my boyfriend was waiting for me, then the ;other guy' told my boyfriend that wed been seeing eachother, for 3 months and that he should know , my boyfriend is devastated and so shocked. I am very upset that the other guy would be so hurtfull to me and my boyfriend. They both left, and tex me all night. The other guy said he hated hiding around.,and my boyfriend said he was shocked. I had been drifting from my boyfriend for a few months and I met the other guy ages ago. I havent been sleeping wiv any of them as it's all been very confusing.,Now wat do I do,.was the other guy right to tell him or should he have shut his mouth?