It's the trend against declension so instead of using "heating", you use "hotting", or instead of total "spending", total "spend". For many it sounds like it's more modern and young. But there are times when using the base word introduces collision with other meanings.
Interesting. My mental association is quaint and old. Where I live, it's the elderly who use the expression "hotting up;" generally in relation to food.
"Hotting up" appears to be a quirk of British journalism, not necessarily a "trend." Not exactly modern and young when the first usage was in 1923 by P. G. Wodehouse.