Feeding releases pheromones, which attracts other members and thus aggregation.

See:

Nature 218, 169–170 (13 April 1968); doi:10.1038/218169a0

Bark Beetle Aggregation : Effects of Feeding on the Release of Pheromones in Dendroctonus and Ips

This beetle is Rutpela maculata of family Cerambycidae

They are not Cerambycids, they belong to the family Meloidae and are parasitic on various bee species. They probably are Mylabris sp. or Hycleus sp., but without any info (locality mostly) it's hard to suggest a name.

Very similar to Mylabris maculosopunctata or Mylabris kodymi or Mylabris variabilis

check this site out: http://insectsofiran.com/others/category/49-beetles.html?start=60

Thanks for the information, it belongs to Meloidae family, but it is not easy to know the species  from the photoes.

But did you read my answer about why they aggregate? :)