One thing I noticed on your tray is that the grain in the layers of the bottom are at right angles. This can cause warping since the individual pieces of wood will expand/contract at different rates with or across the grain. This is why plywood will have an odd number of plies, which will "balance" this warping. I suspect, however, that the major cause of the warping is the direct sunlight on the wood, which dried out the upper layer of the bottom more than the lowest layer. It shrunk more than the other, which caused it to bow upward. If that is the case, then letting it set in a shady spot for a period of time might allow it to flatten again, as the moisture contents become more similar.