Reading the linked report it would appear that the motivation was primarily robbery. Whether the associated vandalism was a targeted attack or merely the corollary of robbery, rather as burglars not only deprive you of your valuables but turn your home over into the bargain, we'll no doubt learn in good time.
Bomber Command memorials are particularly prone to being targeted for despoiling, witness the one in London as well as Harris's statue. I'd be surprised if there were not some added protection in place at Lincoln for the upcoming open weekend, but even if so it was clearly insufficient. A lesson to be learned?