They're not cumulative as that's not the correct term for what you're describing. The squad will take "subsequent" test until they fail the pinning test or they are not shot at with pinning weapons anymore. But it is only one per squad firing at the unit.
Also, from the
FAQ -
http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2170001a_40k_Rulebook_FAQ_Version_1_5_January_2012.pdf
Q: How many Pinning tests can a squad firing multiple
Pinning weapons cause on the enemy it is
shooting?(p31)
A: Each unit can only cause a maximum of one Pinning
test on each enemy unit wounded, per turn, regardless
of the number of wounds caused, unless specifically
stated otherwise.