It's unusual in the sense that, by the 11th century, most Christian countries in western and central continental Europe that weren't in regular contact with Muslim polities had more or less abandoned slavery (or at least began to abandon it) already. At the same time, up to a quarter of Englands population was still enslaved, at least according to the 1086 Domesday Book - it's hard to put into numbers because the records of at least one region completely omit slaves and it is also unclear if they referred to serfs (unlikely, as serfdom continued to exist) and/or counted households or individuals.

Most (Christian) continental regions practiced a form of serfdom instead, except for example Scandinavia (a bit more complex due to not de iure abandoning thralldom until about 1300), Normandy (either due to the 996 peasant revolt or to counteract depopulation), Saxony and Frisia as well as Basque lands. One however has to note that this shift largely only applied to Christians, as Pagans and Muslims continued to be targeted and could be enslaved.

This would generally be caused by the fact that the church, while conflicted, was against the chattel form of slavery, starting with Writers of the Patristic era like Augustinus and Ioannes Chrysostomos, who both considered it a sin or at least induced from sin. Gregorios of Nyssa pretty much straight-up wrote that slavery was not only against God's will but also against human nature. During the Patristic Age pretty much anyone, including people legally considered slaves, could become ordained and preach in the church. With the increased hold on Europe the Catholic church gained after the Saeculum obscurum - partially due to reforms, partially due to increased christianisation - slavery also began being gradually phased out, usually being replaced by serfdom.

The church wasn't exactly keen on just liberating everyone, after all, the Bible still taught people to obey their masters. This caused weird distinctions to be made, resulting in for example the (obviously much later) teaching of Thomas Aquinas, that slavery could be "justified" by personal sin or the belief that while the serfs couldn't be actual slaves in the sense of property, they still had to remain in a form of bondage to their lord. That is also why most of the former slaves in England simply became serfs instead, which meant a few more rights, but still remaining in submission.

For more on the Domesday Book, check out the Domesday Project from Hull University. In regard to Patristics and slavery, check out Preaching Bondage: John Chrysostom and the Discourse of Slavery in Early Christianity or The Unbound God: Slavery and the Formation of Early Christian Thought, both by Chris de Wet.