The likelihood of a mulch pile fire is fairly slim, but it does happen. However, it usually happens at composting and recycling facilities with very large piles (often up to 100' wide x 200' long and much taller than your pile). I do a lot of composting in very large piles, often 8' wide x 20-30' long and 6-8' tall at their max when I've just built new piles in autumn. The longest one I've ever had was about 100' long and we stacked old rotting hay bales four high and three deep. It got really hot and steamed and gave me beautiful compost but it never burst into flame (and that pile was new in February or March of 2011 so it certainly could have flamed up in that hot, dry year, especially since I watered it often so a spark wouldn't catch it on fire.) I've never had a fire, but I have had the piles get beautifully steaming hot inside. That sort of heat is caused by decomposition inside the pile, and it peaks pretty quickly and then rapidly dissipates. You might dig into the pile with a shovel and then find white, ashy looking material but it isn't ash---it is from the decomposition of organic matter and often is fungi or other microbial life that is feeding on the decomposing materials.

I did consult with my son, who has been a professional firefighter for almost a decade, and his only caution is that if you see flames or smoke (not steam) coming from your pile, then turn on the hose and start wetting it down (if you can safely do so) while dialing 9-1-1. He said the one thing you shouldn't do if you think it is on fire is to take any sort of tool and break open the pile or stir it up, as doing so would allow more oxygen to reach the fire and could cause explosive fire growth. When they fight mulch fires, all they do is pour on massive amounts of water to smother the fire. They don't stir up the pile. The smallest mulch pile fire he can remember was a pile roughly 25' x 50' and it took at least two full days of pouring water on that pile to put it out.

Finally, some friends of ours here have had a tree-trimming service dump many truckloads of tree trimming mulch on their property over the years, and it sometimes dumps multiple truck loads in one day, so they end up with really big piles. They've never had a fire. I'm hoping you are as lucky with your mulch pile as they have been with theirs.