Why did the Flightaware groundspeed readout suddenly spike for this flight?
Looking into one set of data from the the popular Flightradar24 service it is not visible.
I think I found the data you're looking at. Since the heading also suddenly changed to (exactly) 360, I'm thinking this is just an invalid data point. Notice that the receiving station jumped between these measurements and something could have happened with the processing in between.
From the same site:

Aircraft with an ADS-B transponder broadcast their identification and this three dimensional position (latitude, longitude, altitude)

This simply implies that this service tries to interpolate the speed from the time between two positions. So say that the clock at the receiving station or something is off (for instance, just an idea), it will just screw up the reading. It comes with the disclaimer:

I believe I witnessed a traffic conflict, altitude deviation, or some other anomaly. Should I report it?
Please don't. Rather, read the terms of use and understand that this data is for casual observation only and not for any operational purpose. FlightAware technology was not designed with the intent to observe safety or regulation anomalies, so please do not try to use it for that purpose.