They tend to do that. On my campaign as Spain, while most of my forces were in Africa (after unifying the Iberian penisular). The French, who were my allies, besieged Pampalona with such a puny forces of peasant archers and sergent spearmen. Then were beaten easily but I didn't want to advance beyond the Iberian penisular into Europe yet, after a while the French came for a cease-fire, when I countered it said very generous, so I asked them to gave Toulouse and they agreed. After a while they besieged Toulouse again, beaten back and asked for a ceasefire, this time I asked for Bordeux and they gave me it again. The AI really does bizzare stuff at times.