The short explanation:
We are moving into a space that will only let us bring about 75% of our shelving units with us. Logically, that means that 25 % or so of the collection has to go so we can fit into where we are going.
The long explanation:
4-year colleges and universities used to use collection volume counts as a measure of the quality of their library. They have a mission to support research faculty, as well as higher levels of student research. Collections are typically weeded to replace volumes that are in bad physical shape, as well as to relieve space problems in certain parts of the stacks. Public libraries tend to weed based on usage - if it has stayed parked on the shelf too long, then it's a candidate for removal, with the exception of "classics" that libraries feel like they need to have around whether they get a lot of use or not.
I see a two-year college collection falling somewhere between these two examples, using some strategies employed by both. But fundamentally, our institutions do not have a research mission, so the need to collect in the same depth as a 4-year institution just isn't there. Further, a book that is 20 years old that has not moved off the shelf in 10 years probably doesn't isn't going to suddenly start circulating like mad. Replacing several of those with a few newer volumes is a better use of resources - especially in our case, when we don't have a lot of extra room to keep adding new materials without weeding out older ones.
Some folks aren't happy that we get rid of old materials. I understand their concerns, and I am always open to their help in identifying titles that we should hang onto and those that are no longer useful. But the bottom line is that we have to weed, especially as we are losing 25% of our available shelving space for the coming year.