This probably won't be very accurate, but a tired battery begins "sulphating", putting out a sulpher gas. When you touch the jumper cables they make spark.........
There are two things I do in an effort to prevent this, and thank my lucky stars, I've never had a battery explosion.
The first is to hook up the jumper cables to the DEAD BATTERY FIRST. This way you won't make a spark at the possibly sulphating battery. Second is to wave your hand or a piece of cardboard or blow air, if a chuck is handy over the battery. This will dissipate most of the accumulated gas above the battery before you hook up the cables which may make a spark.
Won't Deezel be proud of me?
Hope this helps and good luck,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in