Air carrier flight crew reported an altitude deviation due to the aircraft going into vertical speed...

Attributes
ACN	1756247
Time
Date	202008
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Aircraft 1
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Overshoot Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

Descending via the zzzzz into ZZZ there was a small buildup near the ZZZZZ1 waypoint. The captain deviated just a few degrees to the west of it then proceeded back on course via the zzzzz arrival. A few moments later ATC told us to stop the descent immediately because of traffic. I looked up and saw that we were at about 8;580 feet about five miles prior to ZZZZZ2 waypoint. ZZZZZ2 has a restriction of 9;000 feet. We leveled and began the climb back up to 9;000 feet; of which I informed ATC. ATC asked us what happened and the captain explained the situation. By selecting heading for those few degrees to get around the buildup; the airplane went into vertical speed mode for the descent. Neither of us caught it.I was a little distracted by looking at arrival information on my ipad. I just didn't realize we were in vertical speed mode. I know that's what happens; but I just didn't catch it.double check the FMA any time changes are made to the different modes of the autopilot.

Original NASA ASRS Text

Title: Air carrier flight crew reported an altitude deviation due to the aircraft going into vertical speed mode for the descent after deviating for weather.

Narrative: Descending via the ZZZZZ into ZZZ there was a small buildup near the ZZZZZ1 waypoint. The Captain deviated just a few degrees to the west of it then proceeded back on course via the ZZZZZ arrival. A few moments later ATC told us to stop the descent immediately because of traffic. I looked up and saw that we were at about 8;580 feet about five miles prior to ZZZZZ2 waypoint. ZZZZZ2 has a restriction of 9;000 feet. We leveled and began the climb back up to 9;000 feet; of which I informed ATC. ATC asked us what happened and the Captain explained the situation. By selecting heading for those few degrees to get around the buildup; the airplane went into vertical speed mode for the descent. Neither of us caught it.I was a little distracted by looking at arrival information on my iPad. I just didn't realize we were in vertical speed mode. I know that's what happens; but I just didn't catch it.Double check the FMA any time changes are made to the different modes of the autopilot.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.