Décrochage usually means a falling out, dropping out, giving up, stalling (in an economic context that is). Enseigne means corporate name or tradename. The two together would mean "erosion of corporate name" or even "loss of goodwill" (although 'goodwill' in French translates to survaloir or even clientèle). The idea is that the companies being compared are experiencing corporate name attrition.

Decrochage de l'enseigne par rapport a elle-meme: erosion of corporate name with respect to oneself

Decrochage de l'enseigne par rapport au prix marche: erosion of corporate name with respect to the market price