It's because the engine ECU sends data to the ABS/ESP (and back to ECU from ABS/ESP) via the can-bus communication network. MAF readings are used by the ECU to calculate engine load, this info will be sent to the ABS/ESP module.
U codes are faults over the communications network of the vehicle, at a guess it will be 'invalid data received from PCM > ABS/ESP' or something like that. (if there's a fault on ABS/ESP it may give that code as the PCM is getting invalid data from it
And vice versa.
MAF may have been unplugged if someone thought it was faulty, usual test if a MAF is suspect is to unplug to see if performance improves, but on modern vehicles this can put a fault in other systems as you've found out.
£165 for a MAF from DreamScience
http://www.dreamscience-automotive.c...maf-sensor.php