WorldSpan operational info says diverted for mechanical reasons.
From KVS tool:
NRT/0316CT/IN 213P .02E OFF 616P .03L SECU
SFO/0945CT/ETA 916A .21E ETD 1000A .03L MECH
ORD/1020CT/ETA 342P 1.27L MECH DPTS 400P OT
BOS/0659CT/ARVS 720P OT
SFO/DIVERTED FROM ORD TO SFO DUE TO MECH