I saw a lot of Mcp christinae in Mexico (similar to tibicinis but on the other side of the Yucatan peninsula). It seems pretty normal that old pseudobulbs decay. Especially those growing in full sun. The ones that were happiest and most vigorous were shaded by some tree canopy and probably were getting morning sun and afternoon shade. Those in full sun only had the most recent growth alive. The hollow old pseudobulbs are used there as pipes to smoke medicinal herbs.

here are some images of plants in situ,
http://flic.kr/s/aHsjFfY2Px
I have to add more photos but had not much time to edit