For me, there are three idiomatic forms:
1. Using "lag behind" gives a target/reference as a prepositional relationship, not as an object of the verb "to lag".
2. Using "caused to lag" allows one to specify a causal agent, but again not as an object of the verb "to lag".
3. Using "lag" alone is a subject-verb construct, leaving an implicit target/reference from context expectations. A coach or supervisor might scold someone for lagging.
As a bit of a tangent, I actually wonder if the etymology of "to lag" is more Germanic than some people assume. The verb lagern has many uses for placing, storing, and leaving behind. It's where our English concept of a "lager" beer comes from too, referencing the way the beer is fermented in (cold) storage. If this linguistic connection remained fresh, we might think of an SVO construct of lagging as the opposite of the intent in this article. The leader would lag the follower by leaving them behind!
Adding behind after lag as a verb is more of a "because it sounds good", perhaps as a subconscious way to emphasize the verb, but it isn't a grammatical requirement at all.
Leaving it off is almost certainly more to keep the headline short than anything else.
Note also that these aren't really questions of grammar (syntax) but meaning (semantics). Does "lagged" mean the same thing as "trailed" in this kind of construction? It didn't some decades ago, but maybe it does today. Or will tomorrow.