Was it a sudden decision on his part, or had he already made up his mind when he parted from me a few hours earlier? 
( ( ( ( ( ( Was it ) ( ( a ( sudden decision ) ) ( on ( his part ) ) ) ) , ) or ) ( ( had he ) ( already ( made ( up ( ( his mind ) ( when ( he ( parted ( ( from me ) ( ( a ( few hours ) ) earlier ) ) ) ) ) ) ) ) ) ) ) ? )
(ROOT (SQ (SQ (VBD Was) (NP (PRP it)) (NP (NP (DT a) (JJ sudden) (NN decision)) (PP (IN on) (NP (PRP$ his) (NN part))))) (, ,) (CC or) (SQ (VBD had) (NP (PRP he)) (VP (ADVP (RB already)) (VBN made) (PP (IN up) (NP (NP (PRP$ his) (NN mind)) (SBAR (WHADVP (WRB when)) (S (NP (PRP he)) (VP (VBD parted) (PP (IN from) (NP (PRP me)) (ADVP (NP (DT a) (JJ few) (NNS hours)) (RBR earlier)))))))))) (. ?)))
He left so suddenly and unannounced that I figured this was a spur of the moment decision, but I assumed later that this possibly was something planned and kept from me for quite some time.
( ( ( ( ( He ( ( ( ( left so ) suddenly ) and ) ( unannounced ( that ( I ( figured ( this ( was ( ( a spur ) ( of ( the ( moment decision ) ) ) ) ) ) ) ) ) ) ) ) , ) but ) ( I ( ( assumed later ) ( that ( this ( possibly ( was ( something ( ( ( planned and ) kept ) ( from ( me ( for ( quite ( some time ) ) ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (PRP He)) (VP (VP (VBN left) (ADVP (RB so)) (ADVP (RB suddenly))) (CC and) (VP (VBN unannounced) (SBAR (IN that) (S (NP (PRP I)) (VP (VBD figured) (SBAR (S (NP (DT this)) (VP (VBD was) (NP (NP (DT a) (NN spur)) (PP (IN of) (NP (DT the) (NN moment) (NN decision))))))))))))) (, ,) (CC but) (S (NP (PRP I)) (VP (VBD assumed) (ADVP (RB later)) (SBAR (IN that) (S (NP (DT this)) (ADVP (RB possibly)) (VP (VBD was) (NP (NP (NN something)) (VP (VBN planned) (CC and) (VBN kept) (PP (IN from) (NP (NP (PRP me)) (PP (IN for) (NP (PDT quite) (DT some) (NN time)))))))))))) (. .)))