Three major effects:
1) You were forced to take what HST people call a "Stretegic Deconditioning." Without getting too long-winded, when you work out for a long period of time (months), your muscles become resistant to growth from training. When you take time off, it reverses the effect. Generally, the more time off, the more effect is reversed.
When you start training again, your muscles are very susceptible to damage and growth.
also
2) All those neural adaptations you made to help lift more on compound movements...those basically all went away. So you were weaker when you came back, but as your neural strength returns you'll make sick strength gains. This will cause progressive load, which we know causes tons of growth.
3) Metabolisms adapt to intake levels. When you don't eat a lot for awhile, your body goes into "Starvation" mode, and actually lowers your metabolism. The less eaten, the more severe the effect. Upon resumption of normal eating, weight gain is inevitable. If you're purposefully overeating to bulk, then holy cow, look out. This is the idea behind the ABCDE diet plan, which some of you may remember: deliberately go into starvation mode, then rebound at 5000 cal/day for huge gains.
-casualbb
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I'm glad to hear about the gains though Thai.. keep us updated. This almost makes me want to go like, break my leg or something! 
