Because they are coming inside to get away from the cold, and the gaps they use were already there. The first genuine cold snap of autumn is the single most predictable event in North Texas pest control: rodents and crickets that spent the summer living perfectly happily outdoors start testing the building for a way in, and they do not need much of one.
Nothing about your house changed. What changed is that the outside stopped being comfortable.
What comes in, and roughly when
Crickets arrive first and loudest. Populations peak outdoors in late summer, the first cool nights arrive, and they move toward the warm, lit side of a building in numbers. Exterior lighting is the biggest single driver, which is why the same house gets them year after year while the neighbour does not.
Rodents follow as it gets colder. A mouse gets through a gap about the width of a pencil, a rat through one the size of a quarter. Utility penetrations, weep holes, garage door corners and the gap under a poorly sealed threshold are the usual routes.
Spiders become more obvious rather than more numerous. They reach full size in autumn, males wander looking for mates, and they follow their food toward the structure.
Why the fix is not a spray in November
By the time you are seeing them indoors, they are already in. Interior treatment deals with what is in front of you; it does not close the route.
The work that actually stops next autumn happens in spring and summer, and most of it is unglamorous: sealing gaps, fitting door sweeps, cutting back branches that bridge to the roof, clearing harbourage against the foundation, and swapping white exterior bulbs for warm or amber tones near doors.
That last one costs almost nothing and makes a measurable difference to crickets.
Why this is sharper in North Texas
Our autumn arrives as a snap rather than a slide. A run of warm days ending in a sudden cold front compresses what might be a gradual migration elsewhere into a few nights, which is why it feels like everything arrives at once.
Blackland Prairie clay makes it worse. The soil shrinks as it dries through late summer, pulling away from foundations and opening gaps that were closed in spring. The building did not move; the ground did.
Related questions
I only see them in the garage. Is that still a problem?
It is the normal first stage. A garage is the easiest space to get into and the one people notice least, so it works as a staging area. Whether it stays there depends on how well sealed the door between the garage and the house is.
Does one mouse mean there are more?
Usually, yes. Mice are social and breed quickly, and a single visible mouse in autumn generally means a small population using the same route. Seeing droppings in more than one room is the clearer signal.
Do I need to do anything before the cold arrives?
Walk the outside of the house on a bright day and look for daylight: under the garage door, around utility penetrations, at the corners of door thresholds. Anything you can see light through, something can get through.
Related reading
Already hearing something in the walls?
Autumn is when this gets found and winter is when it gets worse, so it is worth dealing with early. Call (214) 585-7145 for a free estimate.