Limp during BJs

So my SO asked me for a blowjob and this would be the second time that I did this for him. During the first time, he said it felt really good but he didn't cum.

During the middle of the second time, he randomly started to go limp, which was odd. He told me that he didn't know what happened and even looked up why he went limp when he got home.

He tells me that he was into it and he doesn't think that it was my fault or that I had bad skills.

Is there a reason why he went limp? Can anyone explain?