It doesn't so much "explode" as "melt".
WH40k: Space Marine does a reasonably good visualisation of this; the 'vents' on the front and side of the cannon leak out the plasma medium.
In any case, gets hot is just 'you take a save', not 'you resolve an attack against yourself'.
If it was the latter, the leman russ executioner would never be damaged when it overheated.
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