Look at your tariffs slider: you have a 70% subsidy on imports, which right now is costing you £6351.1 per day. Combine that with your your daily domestic expenses of £2529.7, and you're spending £8880.8, with an income of only £7061.2.

You mentioned it only happens in certain periods; these are periods when your POPs are buying imported goods en masse, basically on government dime.

I would get rid of the import subsidy altogether unless your POPs are having an extremely hard time meeting their daily needs, because, in addition to costing you all this money, subsidizing imports is going to encourage people to buy foreign goods, which is really going to hamper your domestic industry. I almost always play with reasonably high tariffs; it leads to some belt-tightening for your population early on, but it really pays off when your industrial sector gets strong.

EDIT: this influx of foreign goods is probably also one of the reasons that you're paying £807.1 in industrial subsidies per day to keep your factories afloat: your POPs (and factory owners) aren't buying the goods your factories are producing, because they can get imported goods cheaper.