Answer: If the tulips have already bloomed and the foliage is simply beginning to brown, this is probably the start of the normal ripening process. If the tulips have not bloomed yet, there is a possibility of disease. Most tulip diseases show as either a streaking and mottling of the leaves or as greyish patches combined with poor over all growth and a failure to bloom or deformed blooms. If you suspect disease, you should remove and destroy those bulbs.
Finally, if you've ruled out disease and natural ripening of the leaves, it's possible that wind (or a critter) have caused damage to the leaves. If this is the case, the plants should recover and produce nice, healthy leaves next spring.
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