infrequent
irregular
sporadic
unpredictable
accessory
accidental
accompanying
additional
ado
adventitious
afloat
afoot
aleatory
auxiliary
behind the scenes
by-the-way
casual
causal
causative
ceremonial
circumstantial
conditional
constitutive
contingent
current
decisive
determinative
doing
effectual
etiological
eventuating
extra
few
formative
going on
happening
in hand
in the wind
incidental
inessential
institutive
intermittent
nonessential
occurring
odd
off
on
on foot
ongoing
originative
frequent
mere
small
sudden