Because the shaft(s) in the gearbox are still spinning and the reverse gear typically doesn't have synchromesh to equalise their speeds as it engages.
The shaft(s) are still spinning for one of three reasons:
a) the clutch friction disc has not fully released from either of it's mating surfaces
b) the oil in the gearbox is the wrong grade or quantity
c) you are trying to shift into reverse too soon after disengaging the clutch
Anti-sieze compount (e.g copper grease) on the input shaft splines may cure (a) if the clutch pressure plate isn't faulty and the clutch cable (or hydraulics if used) are correctly adjusted. (b) is unlikely unless the gearbox has been leaking or it's had it's oil changed shortly before the problem began.
bad transmission
if u keep your foot on the gas a little it will grind till u come to a complete stop then it will go into gear smoothly not coming to a complete stop before shifting out of reverse
bad synco rings
Check the fluid in the clutch master cyl if it is hydraulic.
The recommended grind size for a French press is coarse grind.
if an automatic transmission is grinding in reverse it is probably an internal failure. The car will probably need a good mechanic who specializes in transmissions to look at it to be sure. I have seen some cars with a simple linkage problem do this...but its unlikely.
If it is automatic and it grinds, that means you must come to a complete stop before shifting into reverse, or your transmission is no good
Manual transmission only: Probably broke your reverse gear or grind up the gear to much and it won't go into gear. Could need a new clutch or thrust bearing does it crunch trying to get it into gear if yes then its probably the clutch
"Tweens" are HUMANS. HUMANS can grind. So yes, tweens can grind..
im assuming that its an automatic,if so then its just because your transmition is messed up,or your rear gears(not the trans gears)are damaged.
Grind? There isn't a place to grind your Pokemon.
The recommended grind size for a French press when using a fine grind is medium-coarse.