ACR X EXCURSION FROM ASSIGNED ALT. ALTDEV.

Narrative

ZOB HANDLING ARRS INTO PITTSBURG GOT VERY BUSY DUE TO PEAK ARR TIME AND EMER HANDLING OF AN ACFT LNDG IN PIT. CTR STARTED ISSUING HOLDING INSTRUCTIONS TO ACFT LNDG IN PIT. WE WERE GIVEN INSTRUCTIONS TO HOLD AT 9000 FT AS PUBLISHED OVER BELLAIRE (AIR). AFTER ABOUT 2 TURNS WE WERE TOLD TO PROCEED FROM AIR TO HLG AND DSND TO 7000 FT. FO (ME) READ BACK CLRNC TO DSND BECAUSE CAPT WAS GIVING A BRIEF PA EXPLAINING THE UNEXPECTED HOLD. CAPT DID HEAR THE ATC INSTRUCTIONS AND CONFIRMED WITH ME THAT THE NEW ALT TO DSND TO WAS 7000 FT. THIS WAS CONFIRMED USING STANDARD COMPANY MANDATED ALT AWARENESS PROCS. WHILE ENRTE TO HLG AND ESTABLISHED IN THE DSCNT; ATC ISSUED A SPD REDUCTION TO US OF 210 KTS. CAPT RESPONDED AND READ BACK 'SLOWING TO 210 KTS AND DSNDING TO 7000 FT.' CTR ASKED CAPT TO CONFIRM THIS AND HE DID. BY THIS TIME WE WERE AT ABOUT 7800 FT. WE WERE ASKED TO SAY ALT AND CAPT REPLIED 'PASSING 7800 FT.' CTR INSTRUCTED US TO CLB BACK UP TO 9000 FT WHICH WE DID IMMEDIATELY. WE WERE NOT AWARE OF ANY CONFLICT AND RECEIVED NO TCASII WARNING. WE WERE THEN VECTORED IN SEQUENCE FOR LNDG AT PIT ON RWY 10R; WITHOUT FURTHER INCIDENT OR COMMENT. I BELIEVE THIS SIT DEVELOPED DUE TO ACFT SATURATION; MARGINAL VFR WX CONDITIONS AT THE ARPT AND THE PRIORITY HANDLING GIVEN TO THE ACFT EXPERIENCING THE EMER. ATC COMMANDS WERE ISSUED VERY RAPIDLY TO MANY ACFT WHICH ALSO CONTRIBUTED TO THIS INCIDENT.