What causes you to need to remove contact lenses before using allergy eye drops?

Hurts the eye. Actually, the benzalkonium chloride preservative can damage the cornea. This is not usually a concern because the bac gets washed out of the eye before it can cause any harm, but if you use the drops with contacts, the contact lenses hold the preservative onto the cornea for long enough to cause damage.
Drops ruin contacts. All eye drops contain benzalkonium chloride, an agent that prevents bacteria from growing in the medicine bottle once the seal is broken. Benzalkonium chloride damages contact lenses. It makes them cloudy. Contact lenses can be expensive. Put the drops in before you go to bed & your lenses are out. In the morning wait 10 minutes after putting in drops before inserting your lenses. Hakuna matata!