You have this Ultimate Underground Belts capable of underground distance of 155. Between {-7461,98703} and {-7461,98817} there are around 115 tiles of underground section, so when you placed underground belt exit at the {-7461,98728} the whole underground belt part between old underground belt pair had to be squished. When you started building horizontal belt they started to slowly unsquish but not all of them did as it depends on where is the front of line.