Found two of my group 31 batteries in the garage each in a small puddle of fluid today. They were fully charged at the time. One is almost brand new, the other less than 20 discharges. Thought I'd been careful with them. They seem to still have fluid inside. What could this be from? A major issue or does it just happen sometimes?

Overfilling with acid/water will cause that out of the top. 

If the batteries have a cracked case, that could be from being frozen not fully charged.

 

Hm, well I've never opened or filled them. Positive neither has frozen.

My guess was that that they "leaked" when overfilled and then charged.  (Has happened with mine before).

Was the garage pretty hot?

Nope, cool 55-65 all the time. They did get warm in the sun last time I used them though. Then recharged them when I got home. Maybe that was it.