Well, if all five were travelling under the limit, and too close to each other, and the first one jammed the brakes, speed was a factor. The following four did not scrub off enough speed fast enough to avoid crashing. Or the last one did not, and pushed the ones that did forward.
But, and here's the key, a factor is not a cause. The cause would be following too close and/or inattention to driving. Remove those things, and the crash would not have happened.
It's always a factor, rarely a cause. Remove a factor, it changes the crash. Remove the cause, and the crash doesn't happen.