Probably because the incoming random access request is not valid. Therefore the BSC will wait for an Immediate Assignment FOREVER, until the timer t3101 expires.
Whichever the value of t3101 you choose, the timer will always expire, for such “ghost rach”. There is a high amount of ghost rach in E/ BTS, because it is extremely sensitive and does not filter the ul noise it receives. It tends to decode a lot of ghost rach. The positive aspect of this sensitivity is a bigger cell range.
In order to filter the ghost rach, we experimented quite succesfully with the MAXTA and TALIM values.
Try setting MAXTA=20 and TALIM=19 (or even lower MAXTA if you can afford it). This will successfully reduce the impact of ghost rach on your t3101 expiry counter (CESSTIMASS)