First of all, welcome to the forums! Having been through this myself quite a bit, I will say that their are several likely factors:
1) Your tobacco is too moist. You should probably let it dry out a while prior to packing. That may just be guess and test to find out what that optimum dry time might be for that particular tobacco.
2) You could be smoking too fast. If you smoke too fast, you end up creating an ember that's too hot, which then creates moisture beneath the ember, that settles into the bottom of your bowl.
3) You could be a wet smoker, where some excess saliva travels down the stem from your mouth as you smoke, which will make the tobacco wet at the bottom.
4) Excess moisture within the stem could also travel down the stem and into the bowl, making the tobacco wet.
If you notice what's called a "gurgling" sound as you smoke, one or all of these could be to blame. Then again, it may never get quite wet enough to gurgle, yet still be too wet to burn properly. Firstly, I would dry that tobacco more if you do already. Next, watch your cadence and don't smoke too fast. You want to "sip" your smoke, not smoke like a dragon. If your pipe will pass a cleaner during your smoke sesh, run one down the stem to gather up the excess moisture.
Bottom line is, "dottle" happens. It's always going to happen, especially with an aro. Just take it slow, enjoy yourself, and don't sweat the dottle. Lots of guys here have dottle all the time. That last third bowl is sometimes just a filter, or so I've learned. Could always try other methods of packing as well until you find what works best for you. Happy smoking!