There are a few different sides to the situation.

The first thing to realize is the role and power of monasteries in the early middle ages. Lindisfarne, for instance, was not just some backwater place. It was the centre of the cult of St Cuthbert, a popular saint especially in Northumbria (the kingdom of northern England during the time). Monasteries could also be HUGELY wealthy, full of chalices, crosses, decorated books, and other objects. These objects weren't particularly defended, so they were basically easy pickings for the Vikings. Note that vikings were not the only ones who might prey on monasteries or churches; the Staffordshire Hoard may be spoils of war and includes cross fragments that may have come from a monastery, church, or chapel.

On the other hand, though, I don't think there's much particular evidence that they would prey SPECIFICALLY on monasteries for some kind of intimidation or "honour" reasons. Remember, the actual documentary evidence that we have about viking attacks on monasteries (e.g., the Anglo-Saxon Chronicles or depictions of martyrdoms from hagiography) come from other monastic, religious authors who wanted to point out the brutality of these pagan, heathen barbarians. In particular, following from the writings of Gildas a few centuries before, many of these viking-age monks saw the Danes as punishment from God, sent to teach the Anglo-Saxons a lesson for their immorality. That immorality (within and without the monastery) was a feature throughout the period. A letter from Alcuin (at Charlemagne's court around 796, I think) tells the abbot to stop allowing the singing of "heathen songs" in the monastic refectory, and to concentrate only on good, Christian songs.

The other factor is that it's somewhat ahistorical to think about vikings just as violent raiding warriors. They did their fair share of raiding, but were also navigators, traders, craftsmen, and more, all at the same time. Further, the scale and scope of their raiding wasn't necessarily out of line with the levels of endemic violence in Europe during the Anglo-Saxon and Carolingian periods.

So to answer your question, first we have to wonder, "DID the vikings bother massacring defenceless monks" more than any other group, or for a particular reason. They might have, but mostly because it was a safer place to raid, and a particularly rich target. But the scale and focus of viking raids could be overstated, because it's based on sources written BY THEIR VICTIMS.