The communication time of SPI bus is uncertain and disturbed by another unknown thread

I am porting the nuttx to a new ASIC.
the ms5611 is successed on spi bus 4.
I get the right data.

but When I use an oscilloscope to observe the CS pin, the CS could be long as 50us. but the sck is The period of SCK is only 100ns。
There seems another unknown thread disturbed the SPI 。 But how to now which one it is?