Strange, I've only really heard of that happening with ammonium perchlorate based compositions. Did you by any chance use magnesium instead of magnalium? In any case, you may want to try to dry your acetone. It's the root of the problem most likely.
Super odd, for sure. Definitely impure or mistaken chems. Particularly because the starting formula offers the dextrin (5-7%) option, bound with water, of course.
Something's probably off with your chems. I've mixed all of those components together in variable ratios, and bound with both water and organic solvents, and never had this issue or any bad reaction, actually. Kinda boring, which is just fine by me.
Check your chems. And your sources. Mumbs mentioned water in your acetone, but this shouldn't be a major issue with this formulation since it can supposedly be dextrin bound. I'm thinking, in line with Mumble's query, that perhaps your MgAl is actually purely Mg? Or mostly Mg? Dunno. Just seems really odd.
Nonetheless, do not make guesses with energetic mixtures--if it's heating up and acting weird, well, something's not right and you are likely at risk. Sort it out before continuing, please. Be careful.