Optima rates their side terminals for the same ampacity as the top terminals. The source of heating in the pictures seems to be the positive side terminal. Heating is caused by high resistance which can be due to corrosion buildup and/or a loose connection or a bad crimp cable connection.
The picture shows an even circular pattern on the negative side terminal and an uneven pattern on the positive terminal. It appears that a loose connection on the positive side terminal caused localized heating and the insulation on the cable caught fire.
It's a good idea to secure the cables with insulated rings or tie wraps on side terminals to keep them from moving or vibrating loose. There's a number of companies making aftermarket Optima battery install kits with custom trays, battery clamps, and cable clamps just for this problem. Google Optima Battery Installation Kit
My battery guy sold me the Interstate AGM. New technology with the traditional packaging. Same price as the Optima.